new Entity destructor

changed entity id's (entities.length automatically increments)

removed "Entity.prototype.dieNextFrame = false;" because all entities
would share the same value

and moved  "entities.splice" into "Entity.onDeath"

also changed the Entity.remove function because after splicing objects
from the entities array their id's will no longer match up with their
index
Merge pull request #1 from spleen387/master

new Entity destructor