Cleaned (a bit) API views + added separated view for user position
Incremented number of add_booleananswer migration (check commit body)
This migration was in conflict with the previous (0026) migration which added
slug field to thematic model.

If you have any trouble with these last migrations please let me now (github.com/pbellon)
Refactored and strenghtened API & core relations
Refactored the sub-directive choosing in SurveyElement directive