feat(api_formatter): Automatically set the content-length hTTP header

- After yielding a personnalised block with the api_formatter we need to recalculate the content-length and set it in the Headers
feat(model_controller): Adds read and update scopes to the controller

- WARNING: next gem version will not be backward compatible, model_controller has been renamed as models_controller. You will need to change your models controller parent class from ```ModelController``` to ```ModelsController```
- new ```read_scope``` method to override in order to limit the reading scope of the default routes formatted as parameters of the ```as_json``` method of ActiveResource. Defaults to no limit.
- new dynamic ```model_name_params``` method to override in order to limit the writting scope. Defaults to all ```params```
Bump version to 0.1.6