[#632] Add simple write permissions

Add rsr_rest_user_<model_name> permissions for all rsr models. This
requires the running of manage.py syncdb --all to get into the DB.

Add IsRestUserOrReadOnly and set BaseRSRViewSet.permission_classes to use
it. This allows users with the rsr_rest_user_<model_name> permission to
use the "write" methods of the API (POST, PUT, PATCH and DELETE).

Suggested use is to create a Group with the rsr_rest_user_<model_name>
permissions and assigin users that are allowed to write to this group.
[#632] More granular API permissions

Having realised the Up app needs permissions to POST to project_update I
decided to implement a first version of more granular permissions for the
API based on one permission per method per model.

Revise akvo.rsr.management.create_rest_api_permissions() to create one
permission for each of the four "writing" request methods POST, PUT,
PATCH and DELETE.

Add RSRModelPermissions which is based on
rest_framework.DjangoModelPermissions, but overrides
RSRModelPermissions.perms_map to match the perms created by
create_rest_api_permissions().

Set BaseRSRViewSet.permission_classes to RSRModelPermissions
[#632] Merge branch 'feature/632_rest_framework' into release/candidate