Use utility method for team-join display. Fixes BUKKIT-3997

The method to make a string from a collection of strings already exists
and should be used when adding multiple players to a team.
Consider arguments to team leave properly. Fixes BUKKIT-3994

Two checks to argument length were changed to properly consider if the
sender is a player instead of an off-by-one logical error.
Create StaticMetadataValue with substantial speed improvements

Because this was removed from crast's massive metadata overhaul, I'm
adding in my my own little repo because it is cleaner and provided
significant speed and memory improvements.