OS X notifications: Activate terminal when clicked.

 - Backport the 'activate' option from terminal-notifier. This makes sure that
   the notification is removed when clicked and can optionally activate an app.
 - Use the 'TERM_PROGRAM' environment variable to detect the terminal and use
   the appropriate activate option. Supports Apple Terminal and iTerm. Else it
   defaults to no app activation.
Merge pull request #42 from NikolausDemmel/notifications

OS X notifications: Activate terminal when clicked.