Hierarchical Regions

This PR adds a dependency on django-mptt and sets up the Regions used for metadata in a Hierarchical fashion so countries are organized under regions. Screenshot below.

![_example_com](https://cloud.githubusercontent.com/assets/506078/3232447/2d430a5a-f0b4-11e3-9d43-6bdd1fb03bc3.png)

The fixtures are updated to provide this Hierarchical list.

https://github.com/jj0hns0n/geonode/blob/98048240cd773db2ed7c615d16d0a020356a1118/geonode/base/fixtures/initial_data.json#L416