Tweak API of `Promise.promisify`.

Follow the lead of `denodeify` in `rsvp` and `q`: eliminate the magic
behavior of `Promise.promisify` so that adding extra arguments to a
node-style callback is not a breaking API change.  See:
https://github.com/raszi/node-tmp/issues/26#issuecomment-48837683
https://github.com/bower/bower/pull/1403#issuecomment-48784169