Allow hooks to return a promise

[daffl]

I want to be able to return a promise from a hook. Promise return values should not be used so that we can do things like:

userService.before({
    remove: function(hook) {
        // Delete the user from intercom.io first
        return Q.ninvoke(intercom, 'deleteUser', { user_id: hook.id });
    }
});

If you want to modify hook data, just change it after .then:

userService.before({
  remove: function(hook) {
    // Delete the user from intercom.io first
    return Q.ninvoke(intercom, 'deleteUser', { user_id: hook.id }).then(function(user) {
      hook.id = user.id;
    });
  }
});

Errors in the promise will be treated as actual errors.