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
> I think the issue here was that I can't import QtGui until vispy has selected its backend.

Why not. This should be ok. Vispy's first two backends that it tries are PyQt4 and PySide. Together with the the fact that vispy will use a backend that's already imported, a user can just import `QtGui` from where-ever he wants, and there is no need to call