[#584] Replace deprecated perms codenames resolver methods

Django 1.6 deprecates Model._meta.get_add_permission(),
.get_change_permission() and .get_delete_permission(). Instead
django.contrib.auth.get_permission_codename() is provided.

Replace the deprecated methods with get_permission_codename().

Remove akvo.utils.get_rsr_limited_change_permission() and replace uses
of it with get_permission_codename().
[#584] Merge branch 'feature/django-upgrade' into feature/du584_replace_perms_codenames_resolver

Conflicts:
	akvo/rsr/admin.py
Merge pull request #587 from akvo/feature/du584_replace_perms_codenames_resolver

[#584] Replace deprecated perms codenames resolver methods