Estonian local flavor: added business registry code field

I've added the field to validate the Estonian business registry code (äriregistri registrikood).

Although I couldn't find a standard or a legitimate source for the business registry code structure, it seems to have the same checksum method as the personal identification code. Even though its 8 characters long, the basic math would make sense (just imagine the last 3 non-checksum numbers being 0's). After testing a fair bit of known correct codes, the checksum works.

The `EEBusinessRegistryCode` field itself behaves in a very similar manner to the existing `EEPersonalIdentificationCode` field.
I created the `ee_checksum` method there just because `EEPersonalIdentificationCode.ee_checksum` seemed too long, but you are probably right - it would be clearer if it was explicitly called.

As for the docstring - I totally missed that.