release-candidate/0.6 redux

[Jameson Graef Rollins]

So what follows is a patch series for a bunch of miscellaneous patches
that should be included in 0.6.  Most of them were originally part of
the release-candiate/0.6 branch, and they are here rebased on top of
notmuch/master at cb8418784c21155ffea79cce8409a7ea3c546937, which
includes all the multipart and crypto rework.

Some of these patches are cleanup, but some are important bug fixes,
some previously sent to the list by others that I have signed off on,
and some by me that were not previously sent to the list.  I highly
recommend that this entire series be incorporated before 0.6 is
released.

jamie.