Emit events for coverage thresholds

[naganowl]

Currently, when coverage thresholds are not met the terminal outputs the failure result with grunt.log.

It would be ideal to have this linked to an event so something like Growl can hook into it for notification.

[geekdave]

Agreed. This could be part of a more holistic fix for #14 where we could have a generic eventing system hook into any number of "reporters". The reporter could display growl notifications, output to files, or send results to a dashboard, ala #17. I might not have any time to work on this for awhile, so let me know if you're interested in working on a PR.