More aggressive block spawning

Still careful to wait for "ready" so that the engine doesn't get
overwhelmed. There is the risk of deleting an object before it is
ready, but as long as max_on_screen is larger than spawn_per_tick it
should be fine. And that shouldn't really be possible given the
requency of message passing back and fourth between the page thread and
the worker.
Matching Ammo new() with Ammo.destory()
Increases removeObject() memory performance #95

The worker's heap is still growing which is a problem, but this
substantially increases memory performance (as tested with OS X/Chrome)
and changes the crash from a sad tab to a hang in the worker (at
137,000 boxes).