OGM-650 Extracting CouchDB dialect into contrib repo

@emmanuelbernard, @DavideD, this PR 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. So the last seven commits are the interesting ones.

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.

Note that I've used _org.hibernate.ogm.contrib_ and _hibernate-ogm-contrib-couchdb_ as group id and artifact id, respectively. I think it makes sense to reflect the nature of a module being in contrib here. When a module gets "promoted" to inclusion within the main repo/dist, these ids would change, but I think that's fair.

I'll send another PR for _hibernate-ogm_ which removes the CouchDB stuff from there.