give paralyzer drones ground threat targets

switch to passing around unit names, not unit objects in WhatHurtsUnit
and CellValueThreat in targethandler
change attack and raider counters

- attack least guarded targets first, not throw yourself at their
hardest point every time
- reduce attack counter when they destroy a target (and a little, but
not much, when things that can hurt them die)
- don't build outmoded vehicle plant when non-outmoded bot lab can be
built instead
improve assistant/builder allocation
stop iunit storage in assist and defend handlers

- change attacker targetting to attack vulnerable targets first
- pass around ids, not iunits for assistanthandler
- include behaviours in defendees rather than storing the iunit
ensure factories get at least one defense tower

- increase defend a building patrol distance to 200
- use last attack cell if none found
- if a turtle with a factory has no shell, pretend it's much closer than
it is
Merge pull request #34 from zoggop/master

BA: prevent iunit access crashes