To be concise, only the condition must be checked to decide whether
the action would be done. Checking `can_afford` is the responsibility of the action.

This would allow easier analysis of expansion location resource density

Gas would be prioritized over minerals when allocating excess workers.

This relies on Python's falsy/truthy treatment of integers.

This would simplify the build order declaration by instead declaring how many
workers is desired. This works under the assumption that in the early game,
the bot needs to keep making workers.

… possible

… boolean

The commands are not coupled with the build order anyway.
Fixes Dentosal#7

Fixes Dentosal#22

Fixes Dentosal#19

# Conflicts:
#	examples/zerg_rush.py
#	sc2/bot_ai.py
#	sc2/data.py
#	sc2/ids/ability_id.py

# Conflicts:
#	examples/zerg_rush.py
#	sc2/ids/ability_id.py
#	sc2/ids/buff_id.py

…der-support