Second draft of info manual

[david at tethera.net]

In order to get docs both as man pages and as info docs for reading in
emacs, I decided to try converting a couple man pages to pod.

There turned out to a painful amount of infrastructure needed to get
this working, so I didn't get much done on the notmuch-emacs docs.

I think all the infrastructure is in place at this point. I'm probably
not going to do much more work into the actual docs until I get some
feedback on the basic approach; it's clear that the autoconverted pod
files need a bit of tidying.

As far as build depends, this needs only perl for the basic docs
(pod2man is part of perl since a while). To build the info docs needs
pod2texi and makinfo, which are part of recent texinfo releases.