Python/IPython InputHook Integration & Interactive Mode

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?
Can you add the commented-out line:
```
# app.use_app('glfw')  # for testing specific backends
```
This will help users see that other backends work if they don't feel like using the `vispy` command-line options.