cpp: xerces: Correct case of class names

Use CamelCase class names as for the other classes; this also
makes them match the wrapped Xerces-C type names, and also
prevents confusion between std::string and ome::xerces::string.
Maven: add Wagon extension to fix deployment
Merge pull request #743 from melissalinkert/pom-fixes-part-2

Maven: add Wagon extension to fix deployment
Merge pull request #741 from rleigh-dundee/cpp-xerces-class-case

cpp: xerces: Correct case of class names