Adding some code coverage to mocha grunt task

![I have no idea what I'm doing](http://www.openbooktoronto.com/sites/default/files/obt/illustrations/I%20have%20no%20idea.jpg)

https://github.com/pghalliday/grunt-mocha-test#generating-coverage-reports

According to the generated /coverage.html file, we're currently looking at:
**72%** coverage, **488** SLOC.

Coverage numbers look awesome, but [/lib/db/mysql.js](https://github.com/mozilla/fxa-oauth-server/blob/master/lib/db/mysql.js) has a reported **14%** coverage, which is skewing the stats. I'm sure we could tweak the globbing/excludes if that isn't valid file to include in code coverage.