Initial add Groups redux
geonode group security redux
Clean up the groups tests.

* Explicitly import the Group and GroupInvitation classes.
* Add a test case to make sure the token URL route works when a user is invited to a group.
Use is instance vs type when determining class type of an object.
Check group permissions in layer_acls
Check groups in search perms
Fix import in security/auth.py
Commenting out GroupLayer and GroupMap Foreign keys due to circular imports
use geonode.contrib.groups.models.Group vs geonode.contrib.auth.models.Group in layers/views.py
use geonode.contrib.groups.models.Group vs geonode.contrib.auth.models.Group in search/search.py
use geonode.contrib.groups.models.Group vs geonode.contrib.auth.models.Group in security/auth.py
use geonode.contrib.groups.models.Group vs geonode.contrib.auth.models.Group in security/models.py
Add a groups m2m field on the ResourceBase.
Initial support for groups in permissions widget. Return groups in geonode.views.ajax_lookup
Merge branch 'groups-redux' of https://github.com/garnertb/geonode into groups-redux
Check a user's group permissions in objects_with_perm.
Add a post_save hook to create default permissions on new resource base objects.
add set_default_permissions and set_permissions to PermissionLevelImixin
use group.slug in ajax_lookup
Move to using set_default_permissions and set_permissions from PermissionLevelMixin in Layers, Maps, Documents