Fix sender on GMIME 3 (resolve issue #103)

This resolves issue #103 (closed), which appears on some systems using GMIME 3.0.

In short, the combined name and address were being used in the address field on the call to g_mime_message_add_mailbox(), contrary to the official documentation. This somehow worked when it shouldn't. The exact reason for the Schroedinbug-nature of this issue isn't entirely clear — it probably has everything to do with the exact version of GMIME and its dependencies — but at least the bug itself is clear. :)

It feels good to be able to post on USENET again!