[Bleeding] Implement Mojang command fallback system. Adds BUKKIT-5385
Match old alias behavior when migrating.

Previously the alias system would pass all arguments from the alias
to its command(s) implicitly. The new system requires arguments to be
explicitly passed so server owners can have more control over where and
how they are passed. To ensure this isn't a breaking change during the
migration from bukkit.yml to commands.yml we now add the $1- argument
to the alias commands to match the previous behavior.
Readd BlockPlaceEvent for half slab to full block. Fixes BUKKIT-5390
Add method to forget callbacks in AsynchronousExecutor
Handle removing entity while ticking. Fixes BUKKIT-1331

If a plugin causes an entity to be removed from the world while the
world is ticking entities the ticking loop gets out of sync and fails due
to trying to go beyond the end of the entity array. To ensure this doesn't
happen we store the loop position as a field so we can fix it up in the
entity remove method just like the tick method does when it removes an
entity.
Readd diff missed in the 1.7 update.
Hide message from connection throttle on startup. Fixes BUKKIT-5403
Rename Minecraft command permission nodes to meet fallback spec.
Add PlayerSelector.java for diff visibility.
Ignore player selectors if not a Command Block.
Update/improve configuration documentation.
Check if entity is in list before decrementing. Fixes BUKKIT-5404
Updated version to 1.7.2-R0.3 in pom.xml for Beta.
Updated version to 1.7.2-R0.4-SNAPSHOT for development towards next release.
Remove chunks from queue if we don't need them anymore.