Renamed is_airviro_system() in the system module to is_airviro()
Added a concept of Airviro version to the system module
Added a hidden Airviro version attribute to model fields
Made the Model meta class only use fields for the current Airviro version

- Added a _get_current_fields() method to the Model meta class.
- Modified the rest of the package to use the above method, where
  appropriate.
Reformatted according to pep8