Stop considering all banks need a username/login

Some new banks I am trying to integrate (from bankscrap) do not need a username or login the need the number of ID card (https://github.com/bankscrap/bankscrap-ing/blob/master/lib/bankscrap/ing/bank.rb#L19). This could be considered as a login, but another bank connector requires a username and a ID card number (https://github.com/bankscrap/bankscrap-arquia/blob/master/lib/bankscrap/arquia/bank.rb#L16) Which make it hard to implement correctly on the current way of doing in kresus. I suggest then to add the username/login to the customFields (for all the banks, maybe renaming the customFields to fields) and consider that an access is : a password + fields (fields being a structured object).

What do you think @bnjbvr @nicofrand