Allow for either number *or function* to be supplied as a target.

[thyrgle]

Supplying a number is the only supported behavior although it might be useful (in combination with repeat, for example) to also be able to supply a function that is called every time the tween is executed. So for instance:

one_move = Tween(1.0, ball, {"x": random_loc, "y": random_loc})
ball_movement = repeat(one_move)

This would make a ball that randomly moves along the screen over and over again.