Fixed issue saving GenericForeignKey in the admin for Map in Django 1.6.2.

MapAdmin used a custom form (for autocomplete on content object) which also
excluded the content_type and object_id field. This does not work in Django
1.6.2 so the fix that works across versions is to change the model fields
to add editable=Fasle. That ensures they are not displayed when adding or
editing objects in the admin.

Also bumped the minimum Django version to 1.4.10 LTS to hopefully minimise
the number of compatibility issues with later releases of Django.
Bumped version number for release
Merge branch 'release-0.4.3' into develop