added mock to requirements

Hi Zach,

This is my first pull request.  Lately I have been interested in contributing to projects, so I'm looking to get a start.  I added a requirements file to include the mock library, so one may install with pip install -r requirements.txt

Let me know if it's cool if I contribute a little bit to this project, or let me know if you have any ideas for improvement.

Thanks,

- Jim