Sketch of solution to #54

Please consider examples/src/xml2ns0.c as a possible candidate concept to implement a streaming XML-parser on top of expat to read in namespaces defined as XML-Files. Of course as of now it is not complete. Once the concept is approved the next steps to include in the stack would be to
- refactor to an agreed on method naming scheme
- add handling of Aliases for NodeId-shortcuts
- add handling of Arrays, e.g. References
- add a ByteString parser and do some unit tests
- add the missing handcoded UA_TYPE_decodeXML methods
- add code to generate the rest of the pack with our tools
- add the decoders to the VTable
- add a function that makes it possible for any server application to instantiate ns0 from an xml-file