Angular Plugin: Removed isolate scope, added onPin, onUnpin, onTop, onNotTop events.

I removed the isolate scope, it's only hooked up to initialization right now anyway, so it shouldn't need to be bound and you can now apply your own ng-controller on an element with headroom applied. The example I used this is was for hiding a subnavigation when the headroom was unpinned.

I also added in events, although there is probably a cleaner way to do this. this should fix #105