Py3support

[Bellaby]

cStringIO is not available in Python 3. #14

gvis_api.py seems to have this import. It needs to be converted from import cString to from io import StringIO on line 30.

cStringIO is used at line 886.

Or a try exept maybe.

It looks like they updated the for Python 3 support https://github.com/google/google-visualization-python