ENH: Add mesh viewer

Adds a `vispy.plot.mesh` plotter. I tried to simplify `MeshData` a little bit in the process.

Closes #548.

Ready for review/merge.
If this is to be a method, then I'd prefer it to be called `get_vertices`. I *think* this is sort of the practice in other places of the code-base. If not, it probably should be :)