Use a DTD to verify the layout of the XML response

Date format was incompatible with ours because we have millisecond precision. Added fix.

XMLSec could not properly parse the XML file and caused various errors. The workaround is to use a DTD file.
See: http://www.aleksey.com/xmlsec/faq.html (Section 3.2)
Merge pull request #1 from OpenLearningNet/dtd-fix

Use a DTD to verify the layout of the XML response