[B+C] Add conflictsWith method to ItemMeta. Adds BUKKIT-3830.

The Issue:

Currently, there is no way to check if a given enchantment conflicts with the enchantments already existing on an item. This adds an easy method to check this.

Relevant PR(s):
https://github.com/Bukkit/CraftBukkit/pull/1072

JIRA Ticket: BUKKIT-3830 - https://bukkit.atlassian.net/browse/BUKKIT-3830