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
docs: "Getting started" documentation for stbt >= 0.19

This is the documentation that is published at
http://stb-tester.com/getting-started.html -- the source used to be at
https://github.com/drothlis/stb-tester.com but I think that it belongs
closer to the stb-tester sources themselves.

I moved it here, updated it to GStreamer 1.x, and rearranged it to focus
on installing from pre-built packages, avoiding GStreamer-specific
knowledge (in favour of telling people what pipeline to use for each
video-capture device we recommend) until you actually need it to debug a
problem, and removing information that we already provide provide
elsewhere (such as http://stb-tester.com/video-capture.html and
http://stb-tester.com/infrared.html).

In time I hope to move more of the documentation from
https://github.com/drothlis/stb-tester.com to here.
docs/ocr.md: Use local copy of image

So that the whole article + image can be copied over to the website
(otherwise stb-tester.com/../tests/ocr/UJJM2LGE.png won't be a valid
image URL).