implement users following

//...

// follow the given user
router.post('/:username/follow', auth.required, function(req, res, next){
  var profileId = req.profile._id;
  // grab the user from the request and call the follow method.
  User.findById(req.payload.id).then(function(user){
    if (!user) { return res.sendStatus(401); }

    return user.follow(profileId).then(function(){
      return res.json({profile: req.profile.toProfileJSONFor(user)});
    });
  }).catch(next);
});

// unfollow the given user
router.delete('/:username/follow', auth.required, function(req, res, next){
  var profileId = req.profile._id;
  // find user, call unfollow request
  User.findById(req.payload.id).then(function(user){
    if (!user) { return res.sendStatus(401); }

    return user.unfollow(profileId).then(function(){
      return res.json({profile: req.profile.toProfileJSONFor(user)});
    });
  }).catch(next);
});

module.exports = router;