Full screen control (help needed)

This PR adds a real full screen control: click the button (in the full screen example) and enjoy OpenLayers 3 in its full high performance glory. Here's the example:
  http://twpayne.github.io/ol3/full-screen-control/examples/full-screen.html

However, the CSS isn't great:
 * The full screen button needs better styling.
 * The button's text should be set using a CSS `content` style (currently it's [set with a hard-coded string](https://github.com/openlayers/ol3/pull/new/full-screen-control#L5R25)).
 * The button's text should change depending on whether the map is full screen or not, this should be possible with CSS rules.

The branch is [here in the openlayers/ol3 repo](https://github.com/openlayers/ol3/compare/master...full-screen-control) so please push improvements.

P.S. The [same demo with the canvas renderer](http://twpayne.github.io/ol3/full-screen-control/examples/full-screen.html?renderer=canvas) feels faster to me, but there are still a few glitches (probably issue #518).