IE10 CSS compatibility: custom-controls example

How to reproduce:
 * Use IE10
 * Load http://ol3js.org/en/master/examples/custom-controls.html
 * custom control overlays zoom in/out controls

![custom-controls](https://f.cloud.github.com/assets/496153/1094854/c4f57846-16d6-11e3-8797-3d65113682ca.png)