using the fringe to indicate good signatures

[David Edmondson]

After listening to bremner, dkg et al. from Heidelberg, I threw together
a quick patch to see how we might indicate signature validity in the
fringe. The intention is to prompt more discussion - this code is not
ready to ship.

The patch is attached. The result looks something like:
    http://dme.org/data/images/notmuch-signed-fringe.png

Thoughts?

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