New enemy: The stomper

Added a new enemy to the game, the stomper. This enemy is slower than the rest, but it has a special skill, if it sees the player (in the same column or row without any walls in between, even if it is opposite to the direction is moving) it will charge against it and try to stomp it. After a stomp, if it didn't get the player it has 1 second to recover from the hit against the wall. Also be careful... since after a stomp this enemy will be able to start moving to the opposite direction from where it executed the stomp (unlike the other enemies that can't change their movement to their current opposite direction).

Also some bug fixing, added some code to avoid the arrow keys from scrolling the webpage and if you lose, you can restart the game with the Space bar.