Adds Python 3 package support

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=756057

See changelog for details.  Some justification for the package split: pybuild is really the best way to build Python 2 and 3 packages, but it really wants the libraries to be named python-foo and python3-foo.  Thus the splits for those library packages, which doesn't contain the /usr/bin scripts.   Then, because we want to give users the option of running /usr/bin/gunicorn as Python 2 or 3 without name collisions, I gave the Python 3 versions the typical "3" suffix and put them in their own package.  It only then made sense to put things like the examples in a separate, common package.

There's probably some reasonable quibbling around the edges, e.g. should /usr/sbin/gunicorn-debian go in -common?  Seemed to make the most sense to me, although it's a Python 2 script (would be nice at some point to port it).

The .debs should be lintian clean now.

I did not enable the tox tests in d/rules because more work needs to happen to make those work, e.g. working out additional build-deps, and probably hacking a bit on the tox.ini (or its invocation) so that only the relevant environments are run, and no dependencies are downloaded from PyPI.  But I figured this was a big enough change that the tox support can come later.  It bothers me that the tests aren't run at build time, but I don't think this is a regression from 19.0-1 so I'm closing my eyes and plugging my ears for now. :)