Add support for OS X Mountain Lion Notification center

Hi @fnando!

I've recently made the switch to OS X Mountain Lion and really miss my test notifications. This pull request adds that functionality via the [terminal-notifier](http://rubygems.org/gems/terminal-notifier) gem. There are a few limitations: Can't set the app icon, an external gem dependency, but overall I really like it.

Let me know if you'd like me to tweak anything. Thanks!