Color faces for mesh viewer

This will load up a mesh viewer:

```python
import yt
import widgyts
ds = yt.load_sample("MOOSE_sample_data/out.e-s010", step=-1)
dsv = widgyts.DatasetViewer(ds = ds)
dsv.widget()
```

Right now the mesh is just a wireframe.  Having the faces colored would be
awfully nice as well, especially if they could be toggled on and off.
