Added Coveralls badge to Readme

This is a companion to https://github.com/ClusterHQ/build.clusterhq.com/pull/2

This should not be merged until the companion branch is merged and deployed.

Fixes #119 
Although it does clutter up the README file for those old fashioned people who read README files in a text editor rather than via Github. It's a shame there isn't some other way to show these badges.