Fixes #129: Implements sending events to the notification center.

As with growl the user can select in the options whether to send events to the notification center and control whether that happens if the app is in the foreground or always.

With this patch if the user has chosen to only display notifications when in the background and the app is in the foreground the event will not show a popup bubble but will still appear in the notification center history. That can be changed easily.

This code isn't ready to be merged yet, it probably needs to detect if OSX is new enough to use the notification center and my XCode also rewrote the xib file which I need to figure out how to fix but I wanted to get some feedback on whether this is something worth proceeding with.