Implement battler tiers, status weakening

PCs are always Tier 1; enemy Tier is specified in the enemy definition.
Higher-tiered units are given more HP and MP. A few statuses (Ignite and
Protect) that were previously indefinite now wear off on their own.
Additionally, the second-tier elementals have been nerfed (reduced from
80 POW to 50) and the MP multiplier has been changed from 25 to 10, as
PCs were being given far too much MP.