Models and Admin improvements to django-workflows

Changes included in this PR
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- Created initial migration for south
- Added missing file “workflows_tags.py”
- Improve State models:
    + added 'codename' field
    + added 'description' field
    + added 'state_position' field (requires django-positions) to optionally list all states of a specific Workflow in arbitrary order
- TransitionObjectRelation model added to optionally keep track of work flow history of every object; specify the optional parameter "user" in set_state() in order to record the transition
- unit tests updated to take into account the uniqueness of new State.codename attribute
- optional “direction” flag added to Transition
- Improved admin