Implements BE BBAN checksum check

[mhemeryck]

Implements the BBAN checksum check for Belgium.

Following the discussion in #49, I went ahead to implement the BBAN check for Belgian banks.

The check for Belgium itself should be quite simple, see e.g. http://www.credit-card.be/BankAccount/ValidationRules.htm#BE_Validation

The BBAN is represented by XXX-YYYYYYY-ZZ where:

• XXX: the bank code code
• YYYYYYY: the account number
• ZZ: the checksum == XXXYYYYYYY % 97

I also did update the method to add checksums to generated IBANs.

[mhemeryck]

Would also like to check how to extend the BBAN generation as well.