Architecture Changes

changes:
add collidable class to keep track of what can be collided with in the
game
Change killableInGame to a static class - only need one instance
changed from bullet.hit() to every other element incorperating a
hitByBullet() method...bullet cant keep track of everything and what to
do
when hitting specific things but any specific thing can keep track of
that for itself
bullets now have a property called damage
bullets no longer have a hit() method, but a getDamage method which
returns its damage

KNOWN ISSUES:
Removing bullets doesnt seem to be working (this involves the
BasicAmmo.hitbyBullet method and the Collidable.removeCollidable
methods)
Fix shooting bug

no longer able to shoot past attackSpeed

new bug:
when player dies, he keeps dieing