text: Title: No way to copy selected text ?
  Body: Hello,
  
  Kudos for the great job !
  
  I've got some issues though :
  
  Using pywinauto I navigate to an input text.
  Using type_keys (^a^c) I can select the text, and copy it to the clipboard (I can paste it using ctrl+v manually).
  But there doesn't seem to be a way to retrieve it programmatically.
  Or am I mistaken ?
  
  In the doc there is a mention to pywinauto.clipboard, but it doesn't work at all for me :
  
  ```
  >python -m pip install pywinauto --upgrade
  Requirement already up-to-date: pywinauto in python\python36\lib\site-packages (0.6.4)
  ...
  
  >python
  Python 3.6.0a1 (v3.6.0a1:5896da372fb0, May 17 2016, 16:21:39) [MSC v.1900 64 bit (AMD64)] on win32
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import pywinauto
  >>> pywinauto.clipboard
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  AttributeError: module 'pywinauto' has no attribute 'clipboard'
  ```
  
  Any help would be much appreciated, thank you for your time.