feat(ngAnimate): add between callbacks

I'm 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 handle elements positioning / visibility toggling (ie. tooltips, popovers) while still relying on the `$animate.enter`/`$animate.leave` syntax.

Not sure unit testing is required for such a patch, will gladly add them if you want.

cc @matsko