iPython InputHook Integration

First pass integration with iPython 3.x. A new InputHook is defined for VisPy, and it can be enabled from a magic command or by calling the enable_gui() function. A demo is also included.

Caveats:
* It only works with iPython 3.x, I had to get the master repository from GitHub.  This version has many more features for writing input hooks, and the old 2.x API is deprecated.
* I copied some of @rougier's code and used the same documentation.  I guess this needs to be formatted to VisPy's guidelines?

The demo `interactive.py` enables the input hook manually, but I wonder if this should be auto-detected somehow?
I removed a bunch of a back-ends that had more dependencies (e.g Qt, Tk), leaving just a vispy implementation.  I'll add the URL.