Added fix for weird behavior in Voronoi.py. The class variable Edge.EDGE_NUM needs to be reset to zero every time you make a new context so that the edge indices make sense when you re-calculate the voronoi tesselation.
Added ability to change colors of voronoi and the colormap if desired.
Merge pull request #3 from schwancr/master

Edge.EDGE_NUM global variable gets reset in Context.__init__