Made PlayerShip not insta-die when impact with an Enemy. (fixes #24)

* Added a check to PlayerShipImpl#preCollided, only insta-die if collided with something other than an Enemy.
* Made sure the damage taken when colliding with an enemy is not affected by the takeDamageModifier.
* Made sure enemies die when they collide with the PlayerShip. If they are not they will trigger multiple collisions, killing the player.

See #24.