QtCanvas

Adds QWidget subclasses that make it more natural to work with vispy from PyQt applications. The widgets `QtCanvas` and `QtSceneCanvas` each wrap methods from their `Canvas` and automatically embed the native QGLWidget.

Depends on #529
It seems there's likely to be a better way. The user will definitely have already imported PySide or PyQt4, so at this point vispy should be able to select its backend automatically. It would be even better if I could just run `vispy.use(app='qt')`, but that option has been removed.