Performs a Multipart HTTP post to the given URL. A post operation is started
with the creation of a MultipartPost object. Post parameters are sent with
writeParameter() and may be either strings or the contents of an XML file. A
post is finished by calling done() which returns an InputStream for reading
the servers response.
NOTE: This class is meant to communicate with the SBML.org online validator.
As such, it assumes uploaded files are XML and always sends a Content-Type: