Unify interface to DOM writing Writer implementations

All of XStream's XML drivers normally use simply the PrettyPrintWriter to emit XML and do not use the explicit Writers of the implementation. Those can be used to build a DOM out of the written elements. Unfortunately the HSW interface does not define a method, how to access the written DOM tree(s). Therefore introduce a DomGenerator interface and use the JDomWriter's functionality and interface as template, since it has the most advanced functionality.

Note: Personally I was not aware of this use case as it is described in http://www.theserverside.com/news/thread.tss?thread_id=25923 by Joe and I broke the Dom4JWriter in XStream 1.2.