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.

By: mbax <github@phozop.net>

paper-api/src/main/javadoc/org/bukkit/scoreboard/package-info.java

@@ -0,0 +1,6 @@
+/**
+ * Interfaces used to manage the client side score display system.
+ * <p>
+ */
+package org.bukkit.scoreboard;
+