Small add-ons to the RDFa distiller

A small addition has been made on the RDFa distiller service (pyRdfa): there is now a possibility to upload an XHTML file to be distilled, beyond referring to a URI or copying a text to a text box. This feature has been asked for by several users. Both the upload and the text box alternatives use POST, whereas distilling via URI-s uses GET. (Note that the text box version used GET before, which was really not the right way to go...)

Just as for all new services bugs may have been overlooked; contact Ivan Herman if you find any!