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 
If it were just an image, I'd say there is no point having alt text. The text couldn't say the percentage covered and so is useless to anyone unable to see it. However, is this alt text beneficial as a description for the link do you think?

I don't care about the old fashioned people's suffering in this case as long as it makes the document more accessible.