Add Model#revert() to reset changed attributes.

```javascript
var user = User.find(1);
user.then(function() {
  console.log(user.get('email'));             // => bob@example.com
  user.set('email', 'lucille2@example.com');
  console.log(user.get('isDirty'));           // => true
  user.revert();
  console.log(user.get('email'));             // => bob@example.com
  console.log(user.get('isDirty'));           // => false
});
```