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.
Having the user do the reshape actually seems more explicit about what's going on -- the latter hides the underlying (logical if not actual) reshape, and that hiding makes it more implicit to me. From that stance I don't mind having to make them do an extra step.