Changed XML parser (again) among other things

I've been at Europython 2010 all week and have been lucky enough to have time to do some development. As such I've replaced the use of expat (XML parser) with lxml as it seems much nicer to use and is less likely to break if a webdav reply is not quite standards compliant. Hopefully by the end of the week I'll have made other tweaks, improvements and fixes so I can make a proper release. The documentation (which is currently woefully out of date) will not get a real update this week though.