OGM-262 Add integration with CouchDB

@dreab8 worked on the integration of CouchDB.
He has sent the pull request some time ago and because it is now quite busy I had to fix some conflicts caused by the latest changes.

I haven't found an embedded version of CouchDB so this pull request is currently using the initial approach that we adopted for MongoDB. You will need to install CouchDB on your machine and the set an environment variable to build it: COUCHDB_HOSTNAME

If this variable  is not set the integration tests and the couchdb module are not compiled. While writing it now I think we should still compile it and skip  the unit test so that at least everyone can spot compilation errors. I will open a JIRA for this.

I've already installed CouchDB on Jenkins and changed the configuration so the pull request is going to be tested using it.

@dreab8 could you also take a look at it? It would be also nice if you could write some documentation as a separate issue.