Update xml mapping driver and schema to work with embeddables

Some fixes regarding the xml mapping of embeddables. Notably:
 * XmlDriver now also excepts 'embeddable' as root in addition to entity and mapped-superclass
 * Updated schema to reflect addition of embeddables
 * Added test to verify the working the implementation of the XmlDriver

btw, @schmittjoh don't know if a PR on a PR is the most efficient way of contributing to your PR, but I guess it should work :)