Adding and improving the Relax NG

Starting with the original RNG I got from Adrian, I added Relax NG schemas to the repo, and am making changes incrementally to get them to validate, and then to correctly validate the examples.

There seem to have been some problems when the original RNG was automatically generated, but viewing the errors when validating examples in oXygen will, I think, uncover these problems and help get them sorted out.

oXygen is having difficulty with the XInclude commands. I haven't used that in oXygen before, so I don't know how to make the include happen, but I added a fallback which reduces the problem to a warning.