Add support and documentation for running unit tests under multiple Python versions using tox

Summary:
If I can offer my two cents, you gain a lot with the tox/travis/coveralls PRs:

1. contributing devs can easily test on multiple python versions with tox,
2. github integration means you can see in context if any PR will break the tests,
3. you get a "build passing" and coverage badges prominently in README,
4. it's easy and free.

Perhaps more convincingly though, there's no conflict with internal CI. These
PRs are small, self-contained, and will co-exist happily with any internal CI
solution. You can even cross-check if one CI is complaining and the other
isn't, indicating an issue in the test environment configuration.

https://github.com/facebook/facebook-python-ads-sdk/pull/18

Test Plan:
`$ pip install tox`

install python 2.6, 2.7, 3.3, 3.4
Search for and install different versions of python:
https://www.python.org

`$ tox`