Final step for Release 1.2.

Adding a new backend that is already fully operational. The backend is intended to test XML submissions of students and can: - check whether a student's submission is valid XML, - check whether a student's submission can successfully be validated against a given DTD, - check wheter teacher's XPath statements can be successfully run against a model and a student's submission. Directory structure: - backends/xml: Root directory for the XML backend, containing: CHANGES.txt README.txt XML.py - the implementation __init__.py config.py - the configuration file saxon.sh - start script for saxon version.txt xmllint.sh - start script for xmllint - backends/xml/bin*: Containing needed binaries. In this version this is only the latest version of SAXON, downloaded from https://sourceforge.net/projects/saxon/files/Saxon-HE/9.2/saxonhe9-2-0-6j.zip/download This backend has been successfully tested on my own machine, NOT running Zope/Plone, but as a console program. In the upcomming days I'll write unit tests for this backend.