Added new Voronoi example.

Ready for review/merge. Compute the Voronoi diagram of a set of points on the GPU (in the fragment shader). One of the points follows the mouse position and the Voronoi diagram is updated in real-time.

Here is an animated screenshot.

![voronoi](https://cloud.githubusercontent.com/assets/1942359/3777727/2e1d5ec8-1966-11e4-80df-252cd9dfadf8.gif)

Original version proposed by @xoolive.