code: function(groupName, callback) {
          request.head(
              this.getResourceUrl({ path: '/groups/' + groupName, user: null }),
              get404Handler(callback)
          );
          return this;
      }
docs: Check whether a resource group exists or not
  
  @param {String} groupName Name of the group you want to check for the presence of
  @param {Function} callback
  @return {ImboClient}