Added Filter: Post Children

This adds a filter to include/exclude children posts for the Relationship field. When selected (i.e. include children) the parent's title is prepended before the child's title in the relationship box.
![relationship-post_children](https://f.cloud.github.com/assets/12783/1294281/f1ffa13c-307a-11e3-85d7-b8408bb7f16c.jpg)