Add Scoreboard API and Command. Adds BUKKIT-3776, BUKKIT-3834

The implementation is designed around having both a main scoreboard and
numberous plugin managed scoreboards that can be displayed to specific
players.

Plugin managed scoreboards are active so long as a reference is kept by a
plugin, or it has been registered as a player's active scoreboard. Objects
specific to a scoreboard remain active until unregistered (which remove a
reference to the owning scoreboard), but quickly fail if accessed
post-unregistration.