allow customer to define exhibit pages hierarchy (in admin view mode)

* add dropdown box to exhibit page metadata form, so that customer will be able to set parent page and define pages hierarchy

* add exhibit-pages-hierarchy layout (simple stub with list of next level child pages)
![pages-hierarchy](https://f.cloud.github.com/assets/712253/121460/face2062-6d99-11e2-9cfe-5683c32c78bb.png)