Use semi-private API so notifications act like iTunes.

I'm not 100% confident in wanting to get this merged, but I figured it was worth a shot and would be better than me re-patching and re-compiling after every update. :grin:

This PR enables (conditional!) use of the user notification SPI that iTunes does, achieving identical notifications with improved album art and information density.

![screen shot 2014-03-07 at 6 57 07 am](https://f.cloud.github.com/assets/170812/2356638/62b49bea-a5f0-11e3-982c-d1656d75c22b.png)