* [AP_STAGE,FIX] Off-by-one error. Really!?
* [FIX] Freeing DTDs caused some problem I did not understand. Now I keep all DTDs cached.
merge
Added automated support for converting between RDF serializations (also for directories, recursively).

* [FILE_EXT,NEW] Added create_file_directory/1 which ensures a file's directory is there (uses create_directory/1).
* [RDF_SERIAL,EXT] Added all supported file extensions, file types, and file serialization formats.
* [RDF_SERIAL,EXT] If an RDF graph is not modified, but its serialization format is different than that of the file from which it was originally loaded, then the save operation proceeds.
* [RDF_SERIAL_CONV,NEW] New module for converting between RDF serializations.
** rdf_convert_file/3 taken from RDF_AP (now RDF_META).
** rdf_convert_directory/3 added.