Python 3 compatibility

[wojcikstefan]

We should:

• [x] Make sure our client works well in Python v3.x.
• [x] Make Circle run tests on 2.7 + 3 (similar to https://github.com/closeio/tasktiger/blob/master/.circleci/config.yml).
• [x] Add a classifier to setup.py.

[tinvaan]

https://github.com/closeio/closeio-api/blob/master/.circleci/config.yml seems to reference 3.x images now. Can this be closed?

[jkemp101]

Yep, this can be closed. Thanks.