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 don't think you should need these lines. Also, I don't think this will play nicely if as a user I have both PyQt4 and PySide and I want to use PySide, I'd expect to be able to do:

```
from vispy import use
from vispy.app.qt import QtCanvas
use('pyside')
```

But the `use('pyside')` will error out because `vispy.app.qt` importing already forces the `vispy` app system to use `PyQt4`.