Add EntityType wrappers mod on entity registration, for plugins. Fixes #1272

Add the Bukkit EntityType wrappers when the mod registers entities,
rather than when the first entity of that type is spawned. Fixes plugins
which iterate over known entity types through EntityType from the Bukkit API,
expecting a comprehensive list of available entity types.

To do this, we add a new getEntityClass() method to CraftEntity, which is
similar to getEntity() except it looks up a Bukkit wrapper *class* given the
NMS entity class, rather than an creating a new instance. This allows the
entity class to be added in EntityType before any entity instance of that type
is created.

Tested with plugin at agaricusb/IncompatiblePlugin@beac249b9b88b614cf23b44109087dcd0e556a9c
before, 63 EntityTypes were registered on plugin load; after, 365. https://gist.github.com/agaricusb/5793329