Fixed issue: Large method getEntity in CraftEntity.
This commit replaces the 120-line method that handles transitioning from
Minecraft entities to Craftbukkit entites with a 40 line method.
Additionally, this ensures that future entities can be added smoothly
without the need to worry about where in the hierarchy they belong.
Clean up in EntityGenerationTest. Removed many unused imports and added
slightly more useful error messages to catch statements.