text: Title: Lastest release (2017.1.3) dies when trying to display a mock object
  Body: The latest release (2017.1.3) dies with the following stack trace (slightly edited to anonimize it):
  
  ```
    File "/usr/lib/python2.7/bdb.py", line 49, in trace_dispatch
      return self.dispatch_line(frame)
    File ".../site-packages/pudb/debugger.py", line 160, in dispatch_line
      self.user_line(frame)
    File ".../site-packages/pudb/debugger.py", line 381, in user_line
      self.interaction(frame)
    File ".../site-packages/pudb/debugger.py", line 349, in interaction
      show_exc_dialog=show_exc_dialog)
    File ".../site-packages/pudb/debugger.py", line 2084, in call_with_ui
      return f(*args, **kwargs)
    File ".../site-packages/pudb/debugger.py", line 2322, in interaction
      self.event_loop()
    File ".../site-packages/pudb/debugger.py", line 2280, in event_loop
      canvas = toplevel.render(self.size, focus=True)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 1751, in render
      canv = get_delegate(self).render(size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/container.py", line 1083, in render
      focus and self.focus_part == 'body')
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/decoration.py", line 225, in render
      canv = self._original_widget.render(size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/container.py", line 2085, in render
      focus = focus and self.focus_position == i)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 1751, in render
      canv = get_delegate(self).render(size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/container.py", line 1526, in render
      canv = w.render((maxcol, rows), focus=focus and item_focus)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/decoration.py", line 225, in render
      canv = self._original_widget.render(size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/container.py", line 1526, in render
      canv = w.render((maxcol, rows), focus=focus and item_focus)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/decoration.py", line 225, in render
      canv = self._original_widget.render(size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 1751, in render
      canv = get_delegate(self).render(size, focus=focus)
    File ".../site-packages/urwid/widget.py", line 141, in cached_render
      canv = fn(self, size, focus=focus)
    File ".../site-packages/urwid/listbox.py", line 457, in render
      (maxcol, maxrow), focus=focus)
    File ".../site-packages/urwid/listbox.py", line 339, in calculate_visible
      self._set_focus_complete( (maxcol, maxrow), focus )
    File ".../site-packages/urwid/listbox.py", line 704, in _set_focus_complete
      (maxcol,maxrow), focus)
    File ".../site-packages/urwid/listbox.py", line 674, in _set_focus_first_selectable
      (maxcol, maxrow), focus=focus)
    File ".../site-packages/urwid/listbox.py", line 406, in calculate_visible
      n_rows = next.rows( (maxcol,) )
    File ".../site-packages/urwid/widget.py", line 201, in cached_rows
      return fn(self, size, focus)
    File ".../site-packages/pudb/var_view.py", line 120, in rows
      return len(self._get_text(size))
    File ".../site-packages/pudb/var_view.py", line 108, in _get_text
      alltext = var_label + ": " + value_str
  TypeError: cannot concatenate 'str' and 'Mock' objects
  ```
  
  From what I can tell pudb tries to stringify a Mock object. I'm using the [mock](https://pypi.python.org/pypi/mock/) library with Python 2.7. The code works with the previous version (2017.1.2). My pudb config is:
  
  ```
  [pudb]
  breakpoints_weight = 1
  current_stack_frame = top
  custom_shell = 
  custom_stringifier = 
  custom_theme = 
  display = auto
  line_numbers = True
  prompt_on_quit = True
  seen_welcome = e032
  shell = internal
  sidebar_width = 0.5
  stack_weight = 1
  stringifier = type
  theme = classic
  variables_weight = 1
  wrap_variables = True
  ```