Added refactored comment producing using header_comment.

Added refactored comment producing using header_comment.

Added 'omit_empty_tags' option to  XML producer.  Also added some
documentation.

Made SQLite producer produce actual auto incremented fields (using the
INTEGER PRIMARY KEY hack) if the field is an auto-incremented one.  This
modifies slightly how indexes are produced.

Also modified how mk_name is called in the SQLite producer; SQLite
doesn't accept index names that begin with numbers.