tox.ini: Use nosetests and {posargs}

1. {posargs} allows passing arguments or tests on the tox command-line
-- e.g.:

    tox -e py33 -- -v supervisor.tests.test_options

2. Get rid of warnings about unclosed files; hopefully limited to the
tests though perhaps we should check this; note to display these with
nosetests, set PYTHONWARNINGS="d":

    PYTHONWARNINGS="d" tox -e py33

Warnings look like:

    /Users/marca/dev/git-repos/supervisor/supervisor/dispatchers.py:300:
ResourceWarning: unclosed file <_io.TextIOWrapper name='/tmp/foo'
mode='a' encoding='UTF-8'>
  backups=backups,
............................................./Users/marca/dev/git-repos/supervisor/supervisor/dispatchers.py:147:
ResourceWarning: unclosed file <_io.TextIOWrapper name='/tmp/foo'
mode='a' encoding='UTF-8'>
  backups=backups)
................../Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/unittest/case.py:384:
ResourceWarning: unclosed file <_io.TextIOWrapper name='/tmp/log'
mode='a' encoding='UTF-8'>
  function()