Pass callback to hooks so that next can be skipped

/to @bajtos 
/cc @ritch 

See https://groups.google.com/forum/#!topic/loopbackjs/8zcmbxBjYeM

The PR passes the callback func to hooks, for example:

```js
User.beforeCreate = function(next, data, cb) {
// skip next();
cb('Error: invalid email');
};
```