Made it possible to create Edb instances on non-Airviro systems
Added a version property to Edb instances
Made the system module search for current Airviro version

- Changed the LATEST_VERSION attribute to DEFAULT_VERSION.
Made the Model class's serialize methods take an explicit version
Added a version option to the ModelReader and ModelWriter
Merge branch 'topic/versions' into develop

- Remade the Airviro version implementation to handle edb-specific
  versions.
- Made the system module use /usr/aiviro/system/avinfo to find the
  current Airviro system version.