Merge pull request #61 from dgalaktionov/master

Now enemies flash in red when you hit them
Big commit:
Created 2 different controllers, a melee one, and a range one.
Fixed centurion.png transparency.
Added cooldown to enemy hit effect.
Removed some enemies on levels.
Changed some weapons hit damage.
Added support for friendly fire.
Fixed attack cooldown bug.
Replaced hit_by_bullet function by receive_attack.
Inreased bullet collision rectangle size.
Maybe something else I can't remember.
Fixed ranged weapon sounds.