Add mongoDB support and Dockerfile

Added

• MongoDB data store support
• Configure MongoDB URL via CARSHARE_MGO_URL environment variable
• Configure server port via CARSHARE_PORT environment variable
• Created Dockerfile

Changed

• Standardisd ID's on BSON ObjectId
• Overhaul error handling
• Unit test now run twice. First with the fast in-memory data store (fail fast), then as an integration test against a MongoDB docker container