Default Sprite Behaviors: MouseMovementSprite

[pathunstrom]

In the interest of improving the on-boarding ramp for new users and learners, and by extension making our live demonstrations a little more impressive, we'd like to include some Sprites with basic control features.

For this ticket we are defining a MouseMovementSprite. This should be a subclass of TargetSprite with event handlers hooked up to the mouse events to update the target as necessary.

This can be subclassed normally, though probably unnecessary for normal users.