Started work on the Common Driver Library

I started working on a Common Driver Library, which was talked about in #3, and here is my current progress. I built two drivers for the XL MaxSonar-EZ series of sonar sensors, and a couple of basic tests for them. The unit tests currently fail, though, due to issues I have documented in the 2015 RobotPy project. I also built a library to make unit conversions easier throughout the CDL.

Let me know what you think. This is in no way concrete, but more of a sketch of how this might be laid out.
This isn't an OS path, this is a package path. So, '.'.join([base_package, "common_drivers"]) instead.