Add mutate method to model

Add ability to mutate the data directly (i.e. use `$inc` and other MongoDB operators.) This protects data from loss where find -> mutate -> save would lose any transactions in between requests.

```php
    // {... age: 16 ...}
    user = User::one();
    $id = $user->getId();
    $age = $user->age;

    $user->mutate(array('$inc' => array('age' => 1)));
    // {... age: 17 ...}
```

This could have serious effects such as:

```php
    // {... age: 16 ...}
    $user->mutate(array('age' => 1));
    // { age: 16 }
```