Add Couchbase backend

[chedim]

Describe the feature:
We at couchbase developed a backend for JanusGraph that runs on top of a Couchbase cluster and decided to share it with the community.

The backend uses Couchbase cluster's key-value storage to store graph data and FTS service for searches.

Describe a specific use case for the feature:
Running JanusGraph on an existing Couchbase cluster.
Launching JanusGraph on a single database without the need to configure additional search engines like Solr (which is a great product, of course, but not everyone has the know-how to work with it).

[porunov]

That’s really great news! Would be great to check it out.

[mgroves]

I'm looking forward to checking this out, as well, once it's merged :)