Accept alternative ISO-3166-1-alpha2 codes for GB/UK and GR/EL

As state here:

http://publications.europa.eu/code/en/en-370100.htm

> The names of the Member States of the European Union must always be written and abbreviated according to the following rules:
> - the two-letter ISO code should be used (ISO 3166 alpha-2), except for Greece and the United Kingdom, for which the abbreviations EL and UK are recommended;

However in current JSON document, Greece and UK can be found only as GR and GB.

In document which is [linked](http://ec.europa.eu/taxation_customs/resources/documents/taxation/vat/how_vat_works/rates/vat_rates_en.pdf) at Greece definition is used UK and EL instead of GB and GR. So I think JSONVAT client should handle those two exceptions.