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 and to resolve issue #54 .

The basic idea in short:

1) A specific element handler knows exactly which attributes and child elements are valid as well as the exact memory/structure-element whereto we want to decode the specific child.
2) Whenever a element is finished we want to inform it's parent to do some postprocessing, such as setting encoding masks, adding elements to a list, a.s.o.

The basic data structure is a stack (which I implemented as an array with fixed upper bounds, could be list-based as well). 

Of course as of now the sketch is not complete. Once the concept is approved the next steps to include in the stack would be to
- refactor to an agreed-on 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 some missing handcoded UA_TYPE_decodeXML methods, such as UA_Variant
- check if the concept is capable to handle the full variety of the protocol based XML-encodings
- add the rest of the handcoded UA_TYPE_decodeXML 
- add code to generate the rest of the pack with our tools
- add the rest of the possible Nodes such as UAObjectNode to the UA_NodeSet 
- add the decoders to the VTable
- add a function that makes it possible for any server application to instantiate ns0 from an xml-file