Register namespaces to get rid of etree's "ns0" prefix

In current master, etree adds "ns0:" prefix to xlink attributes in GetCapabilities document. This has no effect for direct GetMap requests, but can cause Capabilities-dependent applications such as QGIS to break. I have registered the default namespace and xlink namespace according to http://stackoverflow.com/questions/8983041/saving-xml-files-using-elementtree, and managed QGIS to work with OGCServer service.