Use formencode's xml_compare method to compare test results. This solves test failures on several systems.

@lrowe would you mind doing a quick review of my pull request? I would like to move our Jenkins slaves to the latest Ubuntu 14.04 and currently almost all diazo tests fail.
Yeah. This was why I wanted you to have a look. The problem is that if we want this to work with the lxml parser we have amend/fix the xml_compare method in FormEncode. This means we either have to copy the code and fix it (ugly) or we have to get our fix into FormEncode (probably a lot of work).