Added ArmorChangedEvent

Added an event to fire upon changes to the armor slots. It is posted to
the event bus when onSlotChanged() is called on the four armor slots in
ContainerPlayer.
By this, developers will be able to recognize these changes more easily,
without comparing armor on each tick, and possibly force players to drop armor or call other effects upon equipping.

First time using git from the command line, please don't rip my head off =P