Feature/coveralls

Changes
- Enable Coveralls for code coverage, which automatically runs on all our Travis builds (but __not__ local builds).
- Disable Simplecov.
- Add code coverage badge to readme.
I believe it may be possible to get Coveralls and Simplecov to peacefully coexist if need be.

Notes
- You will no longer get up-to-date coverage in the coverage directory with Simplecov, because Coveralls will handle our coverage in the cloud instead.
- Coveralls has awesome things like history of coverage changes over time and Travis integration.
- The code coverage badge is pointing to master, which does not have these changes yet. As a result, until these changes are merged into master, the badge will display that coverage is unknown.