Allow find to be called anywhere on an Eloquent Model

Currently when you use the query builder on Eloquent models you end up with an instance of the query builder, thus calling find uses the hard-coded `id` instead of the key defined on the model. See issue #1476.

This PR removes the find method from the model itself and instead defines it on the query class. You can now do things like this.

```php
$order = Order::where('seller_id', '=', $seller_id)->find($order_id);
```