[relax-ng] updated relaxng.dtd

After some suggestions from James, I've updated relaxng.dtd to more
accurately reflect the productions in the RELAX NG spec and to make it more
flexible. I also moved the change list to a separate document. See
http://www.snee.com/xml/#relaxng for links to both, and please let me know
of any problems or suggestions.

thanks,

Bob DuCharme          www.snee.com/bob           <bob@  
snee.com>  "The elements be kind to thee, and make thy
spirits all of comfort!" Anthony and Cleopatra, III ii