Simplify "unread" tag handling in emacs UI.

Simplify "unread" tag handling in emacs UI.

This patch is intended to greatly simplify the handling of the
"unread" tag in the emacs UI.  This patch adds a new function
'notmuch-show-mark-read', that removes the "unread" tag in
notmuch-show-mode.  This function is then executed as a
notmuch-show-hook, and by notmuch-show-next-message.  All of the
functions that explicitly marked messages as unread are removed or
renamed.

The idea here is that the user should never have to worry about
manipulating the "unread" tag.  The tag should persist until the user
actually views a message, at which point it should be automatically
removed.  This patch simplifies the notmuch.el quite a bit, removing a
lot of somewhat redundant functions, and reducing clutter in the name
and key-mapping space.