Began some work on allowing individual potions to be used.

Currently, the code denies any positive potion from being used
in a specified region. We can change this to disallow only
specific potions from being used, instead of restricting all
potion types.
Made a method name change.
Removing wither portion reference
Merge branch '0.3' of https://github.com/dualspiral/AntiPotionField into 0.3