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
1. License and copyright in top  plz
2. Headers order and format: self header, separator, C headers, separator, C++ headers, separator, local headers
3. use path from root for local headers, for eg "vehicle.h" -> "vehicle/vehicle.h", "abstractpropertytype.h" -> "vehicle/abstractpropertytype.h"