Fix: NAPinAnnotation and NAPinAnnotationView retain cycle.

It's very real. Those two are holding each-other.  I had to move quite a bit of observer logic around to detangle things.

![leak](https://cloud.githubusercontent.com/assets/542335/2933750/0abd1c6e-d7c3-11e3-8837-8907239ea268.gif)

Closes #31.