Add support for banks with more then 2 auth fields

discover_and_add and update_login method signatures don't allow authentication for institutions with more then 2 authentication parameters. An example is Navy Federal Credit Union (institution_id = 2808).

This PR adds support for more then 2 authentication parameters by changing username, password to *login_credentials, it both maintains the integrity of the old method signature while adding support for additional fields. 

I've left the specs the same to ensure the old method signatures function as previously written. Please let me know if you have any questions.