Set body's user data

This will allow the body to make changes to the object during
collisions.
Remove unused constant
Add a MainContact listener class

This will call the collision funtions of whatever sprites were involved
in the collision.
Add collision functions

These will be called when a collision occurs in the environment.
Update the initialized Platforms

Before this was extrememly difficult to play with the correct jumping
implemented.
Add a basic check to see if player is grounded

This only partly works.  Sometimes collisions are missed, and it also
doesn't account for the player stepping off of a platform without
jumping...To be implemented later.