Kivy Gesture Box

This is more of a Pull Request For Comment than a Pull Request. I don't think it's ready to be merged yet. ;)

I've created an alpha version of a Kivy "Gesture Box" that interprets gestures on the screen and issues an on_gesture event if they occur. If a gesture isn't recognized, the touch event is passed through to any child widgets of the box.

Things that need commenting on:

* I borrowed a lot of this code from Carousel, but I only understand about 85% of it. Please double check that I didn't leave something in there that doesn't need to be there.
* I wrote documentation, but when I run make html, the autobuild.py does not pick up that this file exists. It looks like it's supposed to be automatically picked up, but I must have missed a flag somewhere.
* It passes touches through to child widgets, but it doesn't pass motion. For example, putting a Carousel inside a GestureBox doesn't work. (This is probably a silly thing to do. Less silly would be putting a ScrollView inside a GestureBox)
* Because of that, I haven't actually seen how the documentation I wrote renders in Sphinx
* I didn't do a lot of testing to see how well it works.
* Are there procedures in place to unit test new widgets?