updated abstract
Merge pull request #19 from tjeffrey/master

updated abstract
added availability api
audited readonly (needs more audits). fixed some errors.
changed spec status to bg-draft
Merge pull request #20 from tripzero/master

Availability API and some error corrections
Merge pull request #21 from occops/master

--spit use cases into normative and informative
changed a little bit of the wording of some descriptions
removed nullable marker from vehicle.* attributes.  All attributes should return a valid VehicleInterface.  If the attribute is unsupported, isSupported and available() must reflect that.
Merge pull request #22 from tripzero/master

changed a little bit of the wording of some descriptions
clarified VehicleInterface API

There was no clear correlation between the attributes on vehicle and the data type interfaces.  This update
clarifies how the process of getting and setting vehicle data works.  Attributes on Vehicle are just
interfaces to get/set the data. they correlate with the data type but are not data themselves.

This update also adds the missing VehicleInterfaceError interface.
Merge pull request #23 from tripzero/master

clarified VehicleInterface API
--added group name and url for publishing