Rename ref_id field to ref

Ref is a foreign key, and not a integer representing an id, so it
should not end in '_id'. It is important to change this to ref,
because django automagically exposes the '_id' reference for getting
at the id of a field efficiently. In fact, with this foreign key
called 'ref_id', the database column on audit is called 'ref_id_id' !
Merge pull request #209 from steventlamb/topic/rename_ref_id_to_ref

Topic/rename ref id to ref