Moved vehicles.json and vehicle_parts.json over to data/json directory
Added "type" information to vehicles and vehicle_parts.json files.
Moved initialization logic from game::game() to game::init_data() so that g will be fully initialized prior to installing vehicle parts during vehicle finalization procedure. Added JsonObject loading systems for vehicle parts and vehicles. Vehicles are cached in a temporary object so they can be finalized after itypes are initialized.
Cleaned up unused functions