Use correct method for getting player's team. Fixes BUKKIT-4050

The method getTeam gets the team from name of, as opposed to getting the
team a player belongs to.

This also addresses BUKKIT-4002 and partially BUKKIT-4044
Change perspective of team checking. Fixes BUKKIT-4044

The method's return value was being incorrectly calculated from the
perspective of this.canHurt(that), where it's actually used from the
this.canBeHurtBy(that) perspective.
Disallow dropping items from unauthorized slots