Buff durations

This fix removes the random component from buff durations, which I believe is responsible for most of what people are complaining about with regards to how long buffs last. In addition, cloak and night vision had their duration lowered, eagle eyes time was raised, and resist poison now correctly checks spellpower instead of assuming 100sp for every cast.

At min-max spellpower, Cloak now lasts 2-8.5 minutes, night vision 25-41 minutes and eagle eyes 2-8.6 minutes.