Add notification center support

Similar to and partially based on [pull request 16](https://github.com/salomvary/soundcleod/pull/16).

Although @reaperhulk prefers the notifications to be optional, I think
its ok to have them enabled by default. First, that's what iTunes does
(Apple is always right). Second, it's simpler to implement. Third, it
can still be disabled as all other notifications in system preferences.

This also pulls in a commit by @Wevah for XCode 5 support.