Increased feedback for the user.

Please feel free to ignore this, mainly because it makes the app less "invisible", as it is something you display as a feature of the app.

However, as a user, I many times caught myself thinking "what the hell happened to the song", only to find out there was an ad being blocked. For this reason I made three changes:

* App icon goes red while an ad is being blocked. This is only because I did not have the source vector to the icon, otherwise I would have made an X over the Spotify logo inside the icon and kept it black.
* Replaced the "Active" label in the first drop menu item for the label "Blocking Ad" while an ad is being blocked.
* Added an option to the menu (off by default) to display a user notification (`NSUserNotification`) when an ad is blocked, also displaying how many seconds until music is back.

![screen shot 2014-05-22 at 00 22 51](https://cloud.githubusercontent.com/assets/2136642/3049500/62df9434-e160-11e3-8dbf-b1bad08a8e0d.png)

I hope you find these features useful!