First shot in managing doc with sphinx (cf #59)
In related, use the collection_manager of the model, not the field
Make attach_to_instance and _to_model are easier to inherit from
Add get_field and has_field in RedisModel to avoid using _redis_attr_*
Update README
Merge pull request #63 from twidi/get_field

Define get_field/has_field instead of using _redis_attr_*