Add CloudFlare location provider

Hi there,

This pull request adds a CloudFlare location provider to the UserCountry plugin. 

If Piwik is behind a CloudFlare proxy, all requests have a HTTP_CF_IPCOUNTRY header containing the country code of the visitor as looked up by CloudFlare. This LocationProvider simply uses that value.

I am not sure how to get things set up for internationalization or what your requirements are on that front, so I have just hardcoded English strings for now. If you let me know what needs to be done, I will do it.