Added :scale parameter.
Added pucks.worlds.dev.world12 to demo :scale.

This is the same as pucks.worlds.dev.world6 aside from the reduced
scale and larger screen size.
Improved neighbor-finding algorithm to scale better.

Eddie Pantridge provided a draft implementation, which was revised by
Lee Spector to produce the version in this commit.
Created full, busy, multi-threaded pucks.worlds.dev.world13.

Also fixed click-to-inspect to work properly in scaled worlds.
Developed spawning physics, spawners, and dev.world14.
Tweaks to readme regarding spawning.
Added nursery-threshold parameter.

If there are this many or more active pucks in the world then nurseries
will not generate new pucks.
Allow nurseries to give inventory/memory + ai.world7.
Version bump.