Support additional formats for Atom Publishing Protocol, RFC 5023

I'm working on an implementation of the S-RAMP specification which requires an atom-binding. This binding uses the Atom Publishing Protocol, RFC 5023 (http://tools.ietf.org/html/rfc5023). This part of the atom specification is not yet implemented by the resteasy-atom-provider module.

I have created a patch with the jaxb classes needed to support this feature, along with some tests based on examples from the RFC 5023 specification.