Update tests to use required isConnected() method on VehicleInterface.
Organize a few imports.
Replace a few stray tabs with spaces.
If no extras in VehicleMessage, set extras to null.

This is the simplest way I found to have extras excluded from serialized
JSON when using the Gson library. I spent a few minutes trying to
implement a custom TypeAdapter and TypeAdapterFactory to exclude empty
Collections or Maps, and that was just way too complicated.

Fixed #136.