Added nationality attribute to countries

First of all, thank you for your hard work on compiling this list and make it ruby(autiful)!
I'm planning on use this gem on my next project because it's the only one I found who has proper locale handling.

As I'm gonna also need the nationality attribute for a given country (to acknowledge people's nationalities) I added them to the locales YAML files, so that it can be retrieved the very same way the rest of the country names are. Note though that I've only added nationality for the `en` locale.

I'd like to know your opinion on this and if you find it useful, I'll leave it as a pull request.

There are some minor issues I found (although I haven't read the ISO):
 - There are some missing countries, at least from my knowledge. I.e.: Isn't England a country or is instead a region from Great Britain?
 - There are some missing nationalities for the `en` locale
 - None of the other locales have `nationality` filled
 - I have some doubts against the name of the attribute. I'm not a native English speaker and I found 2 ways to say it: Nationality and Demonym.

Finally, I've created a TODO section on README so that is possible to track what's missing and what's planned to do.

Please give me some feedback when you can because I have some undergoing work that depends on both `carmen` and `carmen-rails` gems.

Keep up the good work!