feat($animate): ensure CSS transitions can work with inherited CSS class definitions

BREAKING CHANGE

ngAnimate addClass / removeClass animations are now applied right away. This means
that as soon as the animation starts the class will be added (addClass) or removed
(removeClass) to the element being animated instead of after the -add-active /
-remove-active animations are completed. This allows for animations outside of
ngAnimate to not conflict with $animate.

This commit introduces beforeAddClass and beforeRemoveClass animation event functions and
executes any addClass and removeClass event functions AFTER the class has been added or
removed (this is opposite functionality of how ngAnimate used to work when performing
JS-enabled animations addClass / removeClass animations). If your animation code relies on
any animations being performed prior to the class change then simply use the new
beforeAddClass and beforeRemoveClass animation event functions.

Finally, when animating show and hide animations using CSS transitions or keyframe animations,
ng-hide-remove doesn't require `display:block!important` for ng-hide-add anymore.
chore(docs): update $animate and ngAnimate docs
chore(docs): refactor/improve new CSS transition code in examples