Added IOC country codes

I build a lot of sport applications in Django and in sports it is normal to show the country codes defined by the IOC. I added support for IOC country codes to your app so i can convert back and forth from IOC to ISO. I don't know if a specific extensions as this one is of your interest, however here is my code.
I don't know if this is the way you would extend your code. If it isn't let me know so i can make the required changes.

Basically i added two properties to your country model:

`Country.ioc_code` returns the IOC code for a country.
`Country.country_from_ioc` is a static method for initialising a country using a IOC code.