Destroy a loaded song and remove it from songs controller

There were 2 issues I wanted to clean up:

1)  A deleted song's `li` element would remain in the songs list.

2)  A song that had been loaded and for which a part had been viewed, had observers attached to the notes collection of that part.  When those observers were triggered they'd put the part in a `save` state, disrupting its capacity to be destroyed.  Destroying a note (removing it from the notes collection) would trigger those observers.

![screen shot 2014-11-09 at 5 41 59 pm](https://cloud.githubusercontent.com/assets/597909/4969718/123dedd2-686a-11e4-9902-08edd9e95c98.png)