Add documentation that we only support Python 2

[travisjungroth]

Add version to setup.py
Add shield to readme
Raise exception if ran on version besides Python 2

[hctareq]

may want to explicitly mention python 2 in the readme?

[travisjungroth]

I added an image which draws the python versions from pypi to the readme. Like on this repo https://github.com/travisjungroth/americano . It won't work until it's pulled in and the new version is pushed to pypi.

If someone looks at the client on GitHub, they'll see that it's python 2 in a way that's pretty standard. If they find it on pypi, it will be visible on that page as well because it's identified in categories in setup.py.