Remove WireCard and protocol implementation

The release candidate does some much-needed spring cleaning! 🌬️
Gringotts is now totally (compile-time) warning free (upto elixir
1.6.5) 🙌

Core

Remove our implementation of the Gringotts.Money protocol as ex_money
implements (and tests!) it for us.
Updated various docs and cleaned up some code smell.

Gateways

1. Refactor Paymill, makes it much more readable and also completes
   all the pending functionality.
2. Remove/Deprecate Wirecard as it does not meet the standards of
   this library.

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.

Add another gateway, TREXLE

Added

• Gringotts now supports Trexle as well 🎉

Changed

• api: Reduced arity of public API calls by 1.
  • No need to pass the name of the worker as argument.

Improve documentation and tests

Added

• docs: Improved documentation - made consistent accross gateways
• tests: Improved test coverage

Initial release, add various gateways

Added

• api: Initial public API release.
• core: Single worker architecture, config fetched from config.exs
• api: Supported Gateways:
  • Stripe
  • MONEI
  • Paymill
  • WireCard
  • CAMS