Add mocha test runner and initial test

Added a test runner as required for #20. Added an initial test for #30.

I wasn't sure what the best way to include mocha/chai were - I've went with Bower as there's already a bower.json file, although package.json would make more sense (and grunt-source is overwriting bower.json on run atm). Also, how can a test task be integrated with the existing grunt-source config?

Should be fairly easy to construct other tests from the existing files in the example directory - would result in a lot of duplicated code though. Not sure what the best way to deal with that is.