Add read-only user option

Trying again :+1: 

Add the ability to make a user read-only, i.e. the user is only allowed to list but not to add, edit or delete.
The default user 'guest' is a read-only user.

The prefix caching in pynipap.py is disabled because there are some issues with it.

Hope to see it added because this is (atleast for me) an important feature!