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
Hmm... maybe you should have `from .backends._qt import QtGui`, then, and users will just have to explicitly do `vispy.use('pyside')` or `vispy.use('pyqt4'` *before* trying to import these classes (to ensure `QtGui` actually exists in that module).