Add docs for contracts

Please add some documentation about the contracts Account, Application, Bank, Pebble, Registration, Shipper, Token: A short description for each one (what it is supposed to do?) and an example data flow, like:

- Device registration (what data is sent, which contracts are involved?)
- Data pumping (which contract receives tha data from devices?)
- Data fetching (which contracts should be monitored for which events?)