Renames tests to examples, data to assets.
Massive changes (0.3.0 dev).

This patch introduces a number of wild changes: a Plugin class which
Assets uses to actually load files, a new StateAttributes object which
is attached to a now (far more generic) State object, a new Timeout
object (from which Callback derives), improved speed for extracting
look_at, perspective, and frustum values from matrices.

There is probably much, much more I'm forgetting (such as how I removed
load_program), but I'll try and be more diligent in the future about
single commits.