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 
Might be worth adding an `:alt:` attribute too...at least that's what I did for Twisted Newsbuilder. See https://raw.githubusercontent.com/twisted/newsbuilder/master/README.rst