Emacs: Replace switch-to-previous-buffer.

A comment in the original post at Emacs
Redux (http://emacsredux.com/blog/2013/04/28/switch-to-previous-buffer/)
suggests using the built-in mode-line-other-buffer, as well as the more
intuitive binding M-O.
Emacs: Use use-package with ido-related materials.

Still some cleaning to do here... not sure about the order and cause and
effect of the different ido-related packages.
Emacs: Use use-package with recentf.
Emacs: Use use-package with save-place.
Emacs: Use use-package with SLIME.
Emacs: Use use-package with smartparens.
Emacs: Use use-package with Org.

Including some massaging, re-organizing, and doc-fixes.
Emacs: Use use-package with mu4e and smtpmail.

This is a big commit, including some other cleanups, like reducing all
of the variables that I set into one setq call, and folding another
mu4e-related configuration back into the mu4e section (oops!).

I would like C-c m to also close all windows, but I can't get that working.