$watchGroup can monitor an array or expression.
We watch both email and password by using the same callback function.
$scope.$watchGroup(['user.email', 'user.password'], function(newVal, oldVal){ console.log(newVal, oldVal); });
One thing need to be noticed that, for input type="email", unless you type '@', the $watch won't get fired.
And also, for init, the value would be undefined.
