Fire HangingBreakEvent for when an hanging entity moves. Fixes BUKKIT-3661
Currently if a painting/item frame were to move (due to a piston or other
means) then HangingBreakEvent (PHYSICS) is not fired. This commit changes
that by firing HangingBreakEvent with the RemoveCause of PHYSICS for when
an EntityHanging attempts to move. Cancelling this in the event of an
obstruction (like an extended piston) will cause the event to be fired
constantly.