Add ScoreboardServer from mc-dev for diff visibility
Add a WeakCollection utility class.

This class is designed to be an invisible layer between a normal collection,
and one that silently loses entries because they are only weakly referencable.
Some operations have additional overhead to be semantically correct, but it
maintains the equals contract for all entries, as opposed to identity.

It does not support the equals or hash code method as it cannot easily have
the transitive and commutative properties.
Implement Scoreboard API. Adds BUKKIT-3776

This implementation facilitates the correspondence of the Bukkit Scoreboard
API to the internal minecraft implementation.

When the first scoreboard is loaded, the scoreboard manager will be created.
It uses the newly added WeakCollection for handling plugin scoreboard
references to update the respective objectives. When a scoreboard contains no
more active references, it should be garbage collected.

An active reference can be held by a still registered objective, team, and
transitively a score for a still registered objective. An internal reference
will also be kept if a player's specific scoreboard has been set, and will
remain persistent until that player logs out.

A player's specific scoreboard becomes the scoreboard used when determining
team structure for the player's attacking damage and the player's vision.