Add support for BBAN credit transfers

[olleolleolle]

This is a reformulation of #101 - with fewer changes in the same Pull Request.

This PR adds support for credit transfer transactions to BBAN accounts.

This improves support for Swedish users who may not have user accounts in IBAN format.

Details

Here's a short list of what changes have been included.

• IBANValidator: an empty IBAN is not checked within this validator, but is checked as a separate concern.
• Account: validate presence of iban.
• Transaction: validate iban or bban being present.
• Transaction gains a few new attributes to support BBAN account numbers, and how to express them.
  • :bban for things like Swedish Bankgiro, in the form 5748964.
  • :bban_proprietary for naming the kind of BBAN account (defaults to BBAN), e.g. BGNR.
  • :clearing_code e.g. SESBA (Sweden Bankgiro Clearing Code).
  • :clearing_bank_identifier e.g. 9900 (clearing number for transactions with BankGiro, Bankgirocentralen (BGC)) or 9960 (clearing number for transactions with Plusgiro).

The files that were created in tests were validated with the proprietary SEB Test Bench.