Specify return is a BeaconInventory.
Add MobEffect <-> PotionEffect conversion methods in CraftPotionBrewer

This moves the conversions to a semi-appropriate class as utility methods, so they can be reused in the beacon code.

As a side effect, this also fixes BUKKIT-4357 and BUKKIT-3653
Implement BeaconPaidEvent. Partially adds BUKKIT-2842
Implement beacon API. Partially adds BUKKIT-2842
Fix calling event, add null-checks
Write out custom beacon effects to NBT
Change custom NBT from Bukkit-Blah to Bukkit.Blah