implemented Rigidbody chain attachment 

- box2d library jar added
- Debug draw mode implemented (press "d" for rigidbody debug mode)
- box2d collision with walls implemented (still a little sticky and needs tweaking)
- rigidbodies are loaded/destroyed upon room entrance/exit
-  created ChainEnd, an Entity class for whatever is connected to the last link of the chain
- swinging chain attack implemented. (press "w" for chain attack)
- explosions added upon enemy deaths
- fixed hero blinking
- extended melee attack range
- chain can be disable through enableChain(), disableChain() in Dungeon