Enhance controls accessibility

This PR supersede #1708. It contains:

 - A css refactoring to make controls share style.
 - @mprins work on the zoom control
 - An adaptation of @mprins work for zoomToExtent & Fullscreen controls
 - A visual refresh for controls tips
 - moving from px to em based sizing/positioning for controls.

Please review

![capture decran 2014-02-19 a 12 04 43](https://f.cloud.github.com/assets/21686/2205774/d01a9aba-9955-11e3-9557-3d14335e077c.png)