in-game hud: enemy ball marking

Related to Issue #25.

All art is temporary - animations also likely to change (currently own ball marker is bouncy, and enemy ball markers are static).

Fixed up bugs with ball marking, as well as implemented system for marking enemy balls as well as a player's own ball, all integrated with networking code.

--Implementation in Scene--
To prevent network duplication (I will try to error check for this in script if I have time, but for now), make sure script (BallMarker.cs) is attached to an object that is created only in a player's game instance (i.e., whatever GameObject contains the network management code).

BallMarker.cs should have prefabs included in  import settings, but if they are not there they should be Ball Marker Prefab and Enemy Ball Marker Prefab, respectively.