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.