Add code coverage tracking via coverage.py and coveralls.io

Coverage.py is configured to measure coverage only on the `./src` folder and to include branch coverage statistics. Results are feed to coveralls, which integrates directly with Travis-ci, when a build completes successfully. Additional coverage.py configuration options can be found at http://nedbatchelder.com/code/coverage/config.html.

Files Added:
* `.coveragerc` - Sets configuration options for `coverage.py`.

Files Modified:
* `Makefile` - Added rule (`coverage`) to initiate code coverage analysis.
* `.requirements.txt` - Added coveralls.io and coverage.py requirements.
* `.travis.yml` - Added step to generate code coverage stats and a post-build trigger to report them to [https://coveralls.io](https://coveralls.io/r/NintenJoe/zol)).