Fix REGON validation.

- There is a special case when the result of modulo 11 division equals 10.
  In this case, the correct checksum is 0. Taken from:

      http://wipos.p.lodz.pl/zylla/ut/nip-rego.html#regon
Merge pull request #62 from krisb78/master

Resolve issue in Polish REGON validation