Move 'connected_type' to P2P_Query

The current way of getting connected posts is rather hard to read:

``` php
$posts_ids = p2p_type( 'posts_to_pages' )->get_connected( $post_id, array( 'fields' => 'ids' ) )->posts;
```

Much more familiar would be:

``` php
$query = new WP_Query( array(
  'connected_type' => 'posts_to_pages',
  'connected_posts' => $post_id,
  'fields' => 'ids'
) );

$post_ids = $query->posts;
```
