tox: add doc environment

Using tox to run Sphinx makes it easier to setup Jenkins since no
setup is needed: Jenkins already runs tox on all the default
environments.
Merge pull request #35 from mgeisler/sphinx-tox

Sphinx tox
zpmlib.commands: move argparser setup function here

We will need to import it when generating Sphinx documentation.
tox: move requirements into doc/requirements.txt

This allows us to cleanly extend them in the future without touching
the tox config.
doc: use sphinx-argparse to include cmdline args

This makes it easy to include up-to-date command line synopsis in out
documentation.