S/MIME support

[Jameson Graef Rollins]

Hey, folks.  This patch adds S/MIME support to notmuch-show.  It's
pretty simple, now that the crypto rework [0] is complete.

I was going to wait to submit this patch until we had a test suite
(ehem, dkg!), but seeing as there has been some other interest
expressed in seeing this feature I'm going to go ahead and send it to
the list in the hopes that it might spur development of the needed
tests.

jamie.

[0] id:"1338057946-29209-2-git-send-email-jrollins at finestructure.net"