support OSX notifications as alternative to growl

I have coded the OSX Notifications support for rerun. It uses [terminal-notifier](https://github.com/alloy/terminal-notifier) gem. And you can use it with two different arguments: `-on` or `--osx-notifications`. If you will use it will disable the growl notifications.

Related to issue: #33

How it looks? See here: ![OS X notifications for rerun gem](http://f.cl.ly/items/1q302F041i2I0j1l113U/Screen%20Shot%202013-06-14%20o%2012.11.40.png)