Eager loading associations with with()


This allows for eager loading of relationships:

```php
<?php

// uses 3 queries, regardless of how many users.
foreach(User::all()->with('Task') as $user) {
   echo $user->Task;
}
```

See http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations for more details on the methodology.