OGM-650 Extracting CouchDB dialect into contrib repo

@emmanuelbernard, @DavideD, this dialect moves the CouchDB dialect into this new repo. I've imported the original history from _hibernate-ogm_ for the _couchdb_ module itself.

All the other stuff (integration tests, modules, docs, dist etc.) I've copied over and stripped down to what's needed for CouchDB only. Nothing special to be said for modules, JavaDoc and distribution. They resemble the previous structures from _hibernate-ogm_, only that they contain the CouchDB module only. For the ref guide I've taken the CouchDB chapter basically as is and added a new preface for Hibernate OGM Contributions reference guide. Please check it out.