Scenegraph reorganization

Reorganization:
- Moved code from old scenegraph/entities to new scene/widgets
- Removed old scenegraph package
- Moved Widget, SubScene, cameras, and systems out of viewbox.py
- Moved viewbox to widgets/

This PR should be reviewed only after #281 is merged.
Should this file be renamed? It seems like everything's been moved to `vispy.scene`, do we still need the `examples/scenegraph` category? I don't think the examples there really worked for me anyway...