Initial commit.
Add events.
Add Renderer.Clear.
Add a higher-level user interface package.

This package sits on top of SDL.  It handles:
	- running code on the main go routine.
	- caching images.
	- panicking on errors (not usually Go-idiomatic, but most are really unhandelable anyway).
	- polling events and delivering them to window-based channels.
	- and eventually more (text, audio, etc.).