Support for the optional authentication code.

See https://docs.easypay.pt/?overview=authentication
Support for the optional authentication code.
Support for the optional authentication code.
Support for the optional authentication code.
Support for the optional authentication code.
Optional code parameter now even more optional. Unit tests.
Cannot call empty on a method return.
Merge pull request #2 from goblindegook/master

Added support for optional `s_code` parameter