Animated zoom

This pull request adds animations to the zoom control and keyboard zoom interaction. It works with all renderers.

Notes relating to the side-by-side example:

Zoom control: only the map who's control is clicked is animated, this is because it is the map that is animated, not the shared view.

Keyboard zoom animation (+/-): *all* maps react because *all* maps receive the keyboard event. Note [this line](https://github.com/openlayers/ol3/blob/master/src/ol/map.js#L212).