Player

Player class added.  A player spawns on game initialization and is controlled using W, A, and D.  Currently, movement is accomplished by directly modifying velocity.  Lateral movement is infinite unless there is a direction change - I didn't implement a stopping mechanism yet on the assumption that we'll be using friction.  As of now, players can jump an infinite number of times - I didn't worry about changing this yet since there are no collisions to prevent the player from falling offscreen.