Add :use_legacy_data option to country_select 

This causes it to use the legacy country data from a yaml file (the current behaviour).  Not specifying this option will now raise an exception.

This is some groundwork for introducing the country data from the worldwide api. This will become the default data set.

I've also created a LegacyCountry model to wrap the yaml data.  This will make swithcing data sets easier, as both can have a model that will quack the same.