Add other bosses.

[lendrick]

[mxnemu]

I added the beetle boss to tower_map.tmx. At the moment it's rolling from one side to the other until it bounces on a wall and gets dizzy for two seconds after that it starts rolling to the other wall. I must still add a visual rotation and make sure that it's invincible while rolling. We also need a new animation of the boss changing from walking to the rolled up status.

Update: The BossBeetle will only take damage if it's currently not rolled up.