Movement command, Animation State

Addresses #6 
Everything works as far as I can tell, but I expect the style will need tons of fixing, since I'm new to javascript. I don't mind making lots of changes, so go ahead and tear this apart.
In addition to the MovementCommand and AnimationState classes, It made sense to me to make a Coordinate struct that represents coordinate pairs ({x: __, y: __}). Of course, we didn't specifically talk about that in the original plans, so I can remove it if you don't like it. 
1. If we're going to put docblocks for each of the member variables, then we should at least specify that their units are steps from the top left corner of the field.
2. For member variable documentation, I think it's okay to leave out the newline before the `@type`.