Fixed a couple of mistakes in the JavaDoc.
Incremented to 2.1.1-SNAPSHOT for development towards the next version.
Fixed a bug causing the entity modifier to always lookup the same world

The entity modifier would try to cache each equivalent converter by its
specific type - i.e. whether or not it converts entities or watchable
collections - but this isn't enough to distinguish entity modifiers 
as they store a hidden reference to the world the entity is supposed 
to be occupying. 

Thus, after the first entity modifier is used for "world", it would 
be subsequently cached for every world ("world_nether", "world_end", 
etc.). The end result was that entity modifiers would only work for 
one world.

This patch fixes this problem by adding a proper equals() method to
the entity equivalent converter class, along with all the other 
equvialent converter classes, in case it should prove to become useful
in the future.