Python 3.x support

[erichiggins]

Goal: Make gaek compatible with Python 3 (while maintaining 2.7 support)

As suggested by #20

Python versions supported by Google App Engine:

• 2.7 (Standard & Flexible)
• 3.6 (Flexible)
• 3.7 (Standard)

Docs for Porting Python 2 code to Python 3