Defend now uses default goal if counters are empty
Expensive counters are now built when teched for

If we haven't teched, but still have resources, then build cheap and start
teching at the same time.
Quick fix of more than sign
Peer review: added comment
Peer review - simplification
Peer reviewed: defined two constants

Might want to change the rules for expanding. If eg our army potential is
really low, then we might still feel the need to expand if we have 2000
minerals. Is that good? Dunno. Very subjective. :)
Merge branch 'develop' into expand_new