Update language codes to reflect changes from RFC3066 to RFC4646

[deveiant]

The language codes in the iso639.rb module are now obsolete:  the RFC that used to define best practices for language codes has been replaced by RFC4646  a new one that removes all of the ambiguity around which code was the correct one to use in any given situation.

Linguistics should be updated to reflect these changes.

Strategy:

• Download and parse  the IANA language code registry, generating a module that encapsulates the language-code registry as part of the build
• Add a deprecation warning to iso639.rb and change the library to use the new module.