Allow fixtures to support Closures

It would be nice if fixture files could support Closures. For example:

``` php
// users.php
return array(
    'AdminUser' => array(
        'first_name' => 'Admin',
        'last_name' => 'User',
        'email' => 'email@domain.com',
        'password' => function () {
            $mylib = new MyLibrary();
            return $mylib->hashPassword('password');
        }
    )
);
```

Thoughts?
