adding the lang::catalog::export_to_files proc for exporting all messages in a list of locales. Making import_from_files always import the en_US locale messages first.
making lang::message::register refuse to register a message for a key that doesn't exist unless it's for the en_US locale. This is a precaution for the case where non en_US locales have somehow corrupted keys or keys that have been forgotten to be added to en_US
