Added country code to paypal urls to help paypal localize their site.

From ticket:
"These basic links seems to be quite easy to change (i.e. https://www.paypal.com/webapps/mpp/home, /signup & /login). All, we have to do is to include country code (ISO 3166-1 alpha-2) after domain name. Even though PayPal does not have localization for the requested country, it will show some defaults (i.e. https://www.paypal.com/fi/signup is in English)﻿.

If the country has several supported languages (like Belgium) it will use some defaults and shows a possibility to change it (signup & login pages have dropdown and this '/webapps/mmp/' shows the options at the bottom right corner of the page)"
I'm not a fan of Rails helpers.

The beef of this functions is to make sure the string that was given as a parameter is a valid country code. So, for example, an util function `is_valid_country?` might be useful for this one. It would be easy to reuse, single purpose function.