code: function (deviceManifestIdentifier, deviceIdentifier, params) {
        var path;
        if (deviceManifestIdentifier) {
          path = this.constructPath(constants.DEVICE_MANIFESTS, deviceManifestIdentifier,
                                    constants.DEVICES, deviceIdentifier);
          return this.Core.GET(path, params);
        } else {
          return this.rejectRequest('Bad Request: A device manifest identifier is required.');
        }
      }
docs: Get all the devices for a device manifest or a particular device identified by `deviceIdentifier`
  @param  {String|Integer}           deviceManifestIdentifier A Robin device manifest identifier
  @param  {String|Integer|undefined} deviceIdentifier         A Robin device identifier
  @param  {Object|undefined}         params                   A querystring object
  @return {Function}                                          A Promise