IBANValidator: More strict checksum checking.

'00', '01' and '99' are not valid checksum values.
IBANValidator: Minor simplification in checksum calculation
IBANValidator: Refactor checksum calculation into a separate method

This is useful when converting legacy account numbers to IBAN format.
Merge pull request #131 from intgr/iban-checksum

IBANValidator: Various checksum improvements