feat(ngAnimate): add between callbacks

We're currently working on the migration of AngularStrap/AngularUI-bootstrap to Bootstrap 3.0 + AngularJS 1.2.0.

We need theses callbacks to be able to properly handle element positioning (ie. tooltips, popovers) while still relying on the `$animate.enter`/`$animate.leave` syntax to leverage CSS animations.

Not sure unit testing is required for such a basic patch, will gladly add them if you want.

cc @matsko