Initial vehicle API support

This pull request adds a vehicle/ subdirectory which adds initial support for a vehicle api based on the w3c automotive business group draft: https://rawgit.com/w3c/automotive-bg/master/vehicle_spec.html

So far only vehicleSpeed is suported and only the "get()" operation.

Related to (but doesn't completely fix) XWALK-865
`GVariant* value;`