Improved libxml2 usage
Made CXMLReader abstract and provided the .NET and libxml2
implementations as subclasses. CXMLReaderLibxml2 now provides the same
functionality as CXMLReaderDotNET but is ten times faster in Mono.
Removed unused (possibly leftover) support for relative XPaths in the
.NET implementation, since CXMLReader can't expose the Navigator anymore
anyway.