Add test coverage tool

[emiln]

I stumbled upon yet another nifty tool that we clearly need. It seems that coveralls is all the rage these days, and it even sports a leiningen plugin to (partially) support Clojure.

Clearly we need this.

[emiln]

This was partially resolved with pull request #14, but I failed to fully integrate it just yet.

Currently:

• Manually running lein cloverage generates a local report (in ./target/coverage or at the directory of your choosing using lein cloverage -o somedir) that you can check.
• Linking coverage to Coveralls doesn't work as their server throws an error. See my issue regarding this.