Debian: Build seperate packages for each Debian/Ubuntu distribution

The Debian repository model cannot serve multiple binary packages which
have all been generated from a single source package.  This means that we
need to create a source package for each Ubuntu release so we end up with
binary packages with different names to each other.

This commit adds the Ubuntu release name to the end of the release field
so the binary deb filenames don't clash.  e.g. we end up with:

    stb-tester_0.19-1_amd64.deb
    stb-tester_0.19-1~saucy_amd64.deb
    stb-tester_0.19-1~trusty_amd64.deb

So the target `make ppa-publish` will now publish packages for both
Ubuntu saucy (13.10) and Ubuntu trusty (14.04).
Debian: Improve determinism of Debian package generation

Ideally, given a particular commit running `make debian-src-pkg` would
generate a byte-for-byte identical debian package independent of when
and where it was run.  This would have helped to avoid issues uploading
the source packages to the PPA to be built for multiple different
Debian versions where it complains if the source package is not byte-
for-byte identical.

This commit at least takes the time stored in the changelog out of the
equation.
Merge pull request #168 from wmanley/multi-ubuntu

Ubuntu PPA: Build packages for both Ubuntu saucy and trusty