Remove the payment worker and introduce the Money protocol

Added

• api: Introduces a Money protocol (#71)
• core: Introduces Response.t (#119)
• development: Adds a useful mix task gringotts.new (#78) to help with adding more gateways!
• docs: Adds changelog, contributing guide (#117)

Changed

• api: Deprecates use of floats for money amounts, check issue #62 (#71)
• core: Removes payment worker, Gringotts no longer routes all API calls through this, nor does it start an application.