Provide a portable mail MIME header decoding function in libphutil

Summary:
See T13232. We currently have a hard dependency on the "iconv" extension to support exactly one call in inbound mail handling if you run your own local SMTP server.

Wrap it and provide an "mbstring" alternative.

Test Plan: Called both methods on some inputs, they appeared to have the same behavior.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20069