add Growl notification helper

This will send a message to Growl when the deploy is complete if installed; ignored otherwise.

I stole the application/branch/target code from the [campfire notifier](https://github.com/westarete/capistrano-helpers/blob/master/lib/capistrano-helpers/campfire.rb) so maybe there's a good way to share that logic, but I couldn't decide on the best way to create the helpers.