Incremental animation commit.

In the process of building the animation "system". This commit is just
to prevent the diff from making me go crazy.

I have a plan for animation:
- What frame to show depends on player input but also can be overwritten
by whatever else is happening (e.g. collision overrides walking)
- What animation to play depends on a variety of factors. Some are as
simple as what your x velocity is, while others can't be derived from
anything besides a keypress.
- With these two ideas in mind, a "desired" animation state will be
computed at every step in the update() function. For example, walking
may set the animation state, then colliding with a wall will overwrite
this, all before it's actually drawn to the screen.

I'm in the process of writing animate() which will be called by any
other action (walking, jumping, falling, colliding, etc.) that does
something worth animating.