Support building Python 3 debs

[msabramo]

and add options for customizability

$ make-deb --help
Usage: make-deb [OPTIONS]

Options:
  --python /path/to/python        Path to Python executable to use (e.g.:
                                  /usr/bin/python or /usr/bin/python3
  --python-version [2.x|3.4|3.5]  Python version
  --test / --no-test              Whether to run python setup.py test
  --postinst-commands TEXT        Commands to put in postinst script
  --help                          Show this message and exit.

$ rm -rf debian
$ make-deb && cat debian/rules
'debian' directory successfully placed at the root of your repository

%:
	dh $@ --with python-virtualenv

$ rm -rf debian
$ make-deb --no-test --python=/usr/bin/python3 --python-version=3.5 \
>     --postinst-commands='mkdir -p /var/log/sm'
'debian' directory successfully placed at the root of your repository

$ grep Pre-Depends debian/control
Pre-Depends: dpkg (>= 1.16.1), python3.5, ${misc:Pre-Depends}

$ cat debian/rules
#!/usr/bin/make -f

%:
	dh $@ --with python-virtualenv

override_dh_virtualenv:
	dh_virtualenv --no-test --python /usr/bin/python3

$ cat debian/smbot.triggers
# Register interest in Python interpreter changes (Python 2 for now); and
# don't make the Python package dependent on the virtualenv package
# processing (noawait)

interest-noawait /usr/bin/python3.5


# Also provide a symbolic trigger for all dh-virtualenv packages
interest dh-virtualenv-interpreter-update

$ cat debian/smbot.postinst
#!/bin/sh

#DEBHELPER#

mkdir -p /var/log/sm

Cc: @bszonye, @theaisforannie