Allow different Deep Zoom XML namespaces.

While the official XML namespace according to the xsd is
http://schemas.microsoft.com/deepzoom/2008, DZIs generated with the
Deep Zoom Composer use http://schemas.microsoft.com/deepzoom/2009,
so this should be supported as well.
Merge pull request #13 from Ventero/fixes

Allow different Deep Zoom schema namespaces.