Handling updates

Implement async Cargo aggregate update when new handling event is registered. Uses the awesome Wisper-Async gem, which is backed by Celluloid. Pretty cool. :rocket: 

The main app url will now redirect to /handling, where you can select new and add a new handling event. Once the handling event is saved to MongoDB, a new event is published and the Cargo is updated and saved asynchronously. If the Cargo is misdirected or unloaded at destination then new events will be raised and messages printed to the command line.