Creates a `Fixture` object that the game uses internally to represent walls, floors, and surfaces. Modifies the Room and Player so that the Room sends a list of Fixtures when the Player arrives, and the writes 'create' json commands to the client for each Fixture. For some reason, the floor Fixture's (x,y) position is way off when rendered. I'm pretty sure the client-side JS isn't converting the server's units to pixels properly (#37)