Start bringing in the Makefile.am from gitsh

It introduces the `Makefile.am` from gitsh, and abstracts it a bit. This
`Makefile.am`, or most of it, could be dropped into gitsh again. How to
use it is documented in `DEVELOPERS.md`.

The whole release process is more consistent and simple: `make release`
to build a tarball, Homebrew, Arch, Debian, HTML (from manpages), and
tag it, pushed to the various repos, and with cleanup. The `release`
target is composed of smaller targets that stack well.