Throw away static initialization of XMLClassDescriptorImpl

Currently XMLClassDecriptorImpl has a static attribute _naming which is statically initialized and used just once in the method toXMLName.

I want to remove this as e.g. JAXB works with a different naming then Castor itself - and also I personally dislike static stuff.

I already changed the code (I'll attach the patch soon) and did some tests and everything works fine.