Added coverage testing via coveralls.io

This is somewhat of a gimmick, but it's possible to get automated test coverage reports via http://coveralls.io - here is an example based on my branch:

https://coveralls.io/builds/140306

You can click on the filenames and see the coverage (and we can also exclude files if needed). Anyway, just wanted to suggest it, but I won't mind if other people think we should leave it out.