Add support for couch databases without authentication

[jvdweerthof]

Allows access to couch without authentication by setting user and/or pass to anything that is boolean false (false, null, undefined etc).

Example usage:

const CouchReplicator = require('couch-replicator-api')

    , replicator = new CouchReplicator(
          'http://npm.nodejs.org.au:5984'
        , false
        , false
        , 'registry'
      )