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.
I'll have to ponder this. I like having the indexing mode be explicit (MeshData is already confusing enough), and I have been thinking about making MeshData a bit more shape-agnostic--in general, it should only care about the total number of items in the array. There are some situations when it is nice to have the option of using higher-dimensional vertex arrays.