Added Creature metadata interface.
Added incomplete entity support to all Extents.

The Bukkit implementation supports the new entity API, but it has
not yet been tested. The Forge implementation does not support the
entity API yet.

At the moment, an UnsupportedOperationException is thrown for
Entity.getState() in some implementations, but use of an exception
should probably not be allowed.

BaseEntity is now an interface. It should not be possible to create
instances of BaseEntity because it may be implementation-specific.