Rare drop update
Added 5 minute delay between drops to prevent grinding

Removed more common mobs from drop list
Added cancel() to the end of DropDelayTask just to be safe
Adds invisible custom names to mobs spawned with a mobspawner

This makes us able to detect they are spawned, and stop them from
dropping a rare item, without sacrificing resources, and you can't see
the custom name in game.
Added new notification, which plays when a player gets a rare drop
Re-added mob types to drop table
I realized the way got random names is incorrect

As I use ordinal, and getLength, it has a possibility of returning 1
greater than the highest value. I am now using getLength - 1, which
should solve any problems.
Merge pull request #260 from sabersamus/master

Rare drop update