Implementation of Voronoi diagrams

I've been working on an implementation of Voronoi diagrams in
matplotlib. It is based on the Delaunay triangulation and simply creates
a PatchCollection of Voronoi cells.

I added a function/method to the pyplot module and the Axes class,
respectively, which is called like this:

voronoi(X, Y, ...)
voronoi(X, Y, C, ...)

where X, Y are coordinates of an unstructured grid and C an optional
mappable scalar.