Add coveralls badge to doc site and README.md.

Testing Done:
Sample doc site publish here - you may need to refresh: http://pantsbuild.github.io/jsirois/index.html
And README.md renders as such: https://github.com/jsirois/pants/blob/d5e2a149c2fc368327524d8c7b16168a8936c5db/README.md

Bugs closed: 369, 544

Reviewed at https://rbcommons.com/s/twitter/r/972/