New terminal-notifier formula

The `terminal-notifier` utility is analogous to Growl's `growlnotify` CLI tool. 

To use the program as distributed, one must call the inner binary of an app bundle (due to a library-linking quirk the author notes, in https://github.com/alloy/terminal-notifier/blob/master/README.markdown) – this formula sidesteps this awkward mode of execution with an exec script (see L#15).