Magic Hunter fix and optimization

There was a small bug in magic hunter, where it tried to apply an
enchantment level between 1 and the Enchantment.getMaxLevel(), however
it didn't take into account Enchantment.getStartLevel(), so when it
tried to apply an enchantment level below the start level, an
IllegalArgumentException was being thrown and the magic hunter event
wasn't happening.

Also, it's potentially inefficient to recalculate which enchantments
are possible for each ItemStack every time, so I added a HashMap to
cache the possibleEnchantments for each material type, then check this
cache instead of just looping to regenerate the list each time.