LASERS COLLISIONS WORK NOW

Lasers now disappear on collision with any entity. They were made
massless by making their fixtures "sensors" instead of normal fixtures.
Collision callback functions beginContact and endContact were created
for Entity in order to manage interactions with all pairs of entities. A
getID function in Entity was created for convenient switch statements.
The sound that the projectile makes was temporarily removed... because
it's annoying.