emacs tagging cleanup

[Jameson Graef Rollins]

This is a rework of the series [0].  It addresses some of the comments
from Dmitry, and extends the scope to clean up the tagging interface
in both search and show mode.

[0] id:"1333354853-25729-1-git-send-email-jrollins at finestructure.net"

The goal here is to present a cleaner tagging interface to the user.
We want things to be simple, not confusing or redundant.  To that end
I have extended the notmuch-tag function to handle prompting if
needed.  The main user-facing functions are now:

  notmuch-search-tag      tag thread or region
  notmuch-search-tag-all  tag all thread in search buffer
  notmuch-show-tag        tag message
  notmuch-show-tag-all    tag all message in show buffer

I think this provides a much cleaner interface that is more useful to
users.  In particular, this improves the ability to tag regions in
search mode.

I have eliminated two user-facing functions (notmuch-search-tag-thread
and notmuch-show-tag-message) since they are now redundant.  This
might cause a slight hiccup for those using those functions, but I
think it's better in the long run.

Unfortunately something in the last patch (last two patches, I guess)
is now causing a two of our more esoteric tests to fail.  After
spending half the day trying to figure out why I'm at a loss.  And I
don't want all this work to be wasted so I'm submitting it anyway.
Hopefully someone (maybe the creator of those tests?) can figure out
what's going on.

jamie.