I just committed the semantics that Keith wanted for the 'x' and 'X'
bindings when viewing a thread, (that 'x' archives the thread, and 'X'
clears the "unread" tag then archives).

This is very different than the old behavior of 'x' which was simply to
kill the current buffer without changing any message tags, (that
behavior was and still is available on the 'q' keybinding).

Before I made this change, I implemented what I think is a much nicer
help system for the emacs notmuch interface. It's available on the '?'
key which is hopefully intuitive enough. Please take a look and let me
know if it's easier to learn how to use the various notmuch support
within emacs with this new help mode.

-Carl