Mac Notification Center

Depends on @worrel's PR, simply because I'm using SBT 0.13 and Java 8, and the project as currently in master does not support either.

This PR adds support for Mac's Notification Center via the excellent [`terminal-notifier`][https://github.com/alloy/terminal-notifier] utility.  You will need the command installed if you want to use this plugin (hint: `sudo gem install terminal-notifier`).