convert debian packaging to dh-elpa

[David Bremner]

The main part which needs sanity checking is 

    [PATCH 1/3] emacs: generate notmuch-pkg.el

It may make sense later (after we drop support for emacs23) to make
our standard install also use package.el. 

This is mainly for information; you can see it drops a lot more than
it adds.  People maintaining packages for old derivatives might have
to revert this commit

[PATCH 2/3] debian: convert to use dh-elpa

Initially I thought about doing just a debian upload, but I'll
probably do another point release with some emacs bugfixes from Mark.

[PATCH 3/3] debian: changelog stanza for 0.23.1-2