text: Title: Codec error when running Marvel Tutorial with Python 2 Kernel
  Body: To reproduce, execute marvel tutorial notebook with a python 2 kernel. 
  
  The command:
  `plotter2.plot(unique_coappearences, heroes)`
  
  Produces the error:
  
  ```
  ---------------------------------------------------------------------------
  UnicodeEncodeError                        Traceback (most recent call last)
  <ipython-input-10-7baa7f94961d> in <module>()
  ----> 1 plotter2.plot(unique_coappearences, heroes)
  
  /usr/local/lib/python2.7/site-packages/graphistry/plotter.pyc in plot(self, graph, nodes, name)
      310         if (api_version == 1):
      311             dataset = self._plot_dispatch(g, n, name, 'json')
  --> 312             info = PyGraphistry._etl1(dataset)
      313         elif (api_version == 2):
      314             dataset = self._plot_dispatch(g, n, name, 'vgraph')
  
  /usr/local/lib/python2.7/site-packages/graphistry/pygraphistry.pyc in _etl1(dataset)
      303                   'key': PyGraphistry.api_key()}
      304 
  --> 305         out_file = PyGraphistry._get_data_file(dataset, 'json')
      306         response = requests.post(PyGraphistry._etl_url(), out_file.getvalue(),
      307                                  headers=headers, params=params,
  
  /usr/local/lib/python2.7/site-packages/graphistry/pygraphistry.pyc in _get_data_file(dataset, mode)
      274             with gzip.GzipFile(fileobj=out_file, mode='w', compresslevel=9) as f:
      275                 if sys.version_info < (3,0) and isinstance(json_dataset, str):
  --> 276                     f.write(json_dataset)
      277                 else:
      278                     f.write(json_dataset.encode('utf8'))
  
  /usr/local/Cellar/python/2.7.11/Frameworks/Python.framework/Versions/2.7/lib/python2.7/gzip.pyc in write(self, data)
      239 
      240         if len(data) > 0:
  --> 241             self.fileobj.write(self.compress.compress(data))
      242             self.size += len(data)
      243             self.crc = zlib.crc32(data, self.crc) & 0xffffffffL
  
  UnicodeEncodeError: 'ascii' codec can't encode character u'\xc1' in position 10735680: ordinal not in range(128)
  ```
  
  Pygraphistry version is 0.9.28
  