organisation: add models for network groups

Network groups are either local groups or chapters and these models
represent both. They both contain country groups and region groups
within countries. Views will have to do much of the grunt work to
separate national groups and region groups.
requirements: add django-geoposition

We use version 0.1.5 of django-geoposition which provides a
GeopositionField for longitude and latitude along with an admin
widget allowing users to select coordinates via Google Maps.
Merge pull request #111 from okfn/networkgroups

Network groups