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/506078/3232351/356682bc-f0b3-11e3-981a-4286de323a29.png)

The fixtures are updated to provide this Hierarchical list.

https://github.com/jj0hns0n/geonode/blob/98048240cd773db2ed7c615d16d0a020356a1118/geonode/base/fixtures/initial_data.json#L416