feat($ionicScrollDelegate): add scrollTo(left,top,animate) to delegate

Moves calling of $ionicScrollDelegate.register to $ionicScroll controller, and makes `<scroll>` and `<content>` directives be registered to the scrollDelegate through the $ionicScroll controller.

Also changes ionicScrollDelegate.register to take in a scrollView as a parameter.

@adamdbradley opened PR just so you could look at the changes to ionicScrollDelegate.