Improve installation of emacs mode.

Improve installation of emacs mode.

1) Add a separate targets to build and install emacs mode.

2) Don't hardcode the installation directory, instead use emacs'
   pkg-config module.

3) Install a byte compiled version of the emacs mode.

4) Install the emacs mode in emacs' site-lisp directory.  Put
   "(require 'notmuch)" in your .emacs to load it automatically.

5) Ignore byte-compiled emacs files.

Signed-off-by: Jeffrey C. Ollie <jeff@ocjtech.us>
Reviewed-by: Ingmar Vanhassel <ingmar@exherbo.org>
Reviewed-by: Keith Packard <keithp@keithp.com>