translation of countries in form refs #317
create an helper + twig filter to show translatable string in current locale.

The twig filter is localize_translatable_string

Example : {{ person.nationality|localize_translatable_string }}

The helper may be called with $container->get('chill.main.helper.translatable_string'). The main function is ->localize(array $strings)

Example:

$container->get('chill.main.helper.translatable_string')->localize($country->getName()); #return the name in current locale

close #299