Vdb visitor and ddl sequencing forking

* First version of the Vdb Node Visitor for exporting vdb nodes from modeshape

* Temporarily forks the ddl sequencer from modeshape repository but does not actually put the source into this repository. Instead, uses maven to checkout a copy of the modeshape repo and add the source at build-time. Allows editing of source and copying changes back to repo then pushing upstream in a PR. 
You mention that the fork "allows editing of source." Shouldn't the lexicon changes here, and CND changes elsewhere, be done that way then?