code: function(usernames, rolename) {
      if (_.isEmpty(usernames)) {
        return Promise.reject(new Error('One or more usernames must be provided'));
      }
  
      if (!_.isArray(usernames)) {
        usernames = [usernames];
      }
  
      return Role.findOne({
          name: rolename
        })
        .populate('users')
        .then(function(role) {
          return User.find({
            username: usernames
          }, {
            select: ['id']
          }).then(function(users) {
            users.map(function(user) {
              role.users.remove(user.id);
            });
            return role.save();
          });
        });
    }
docs: remove one or more users from a particular role
  TODO should this work with multiple roles
  @params usernames {string or string array} - name or list of names of users
  @params rolename {string} - the name of the role that the users should be removed from