tox.ini: Use nosetests and {posargs}

* `{posargs}` allows passing arguments or tests on the tox command-line -- e.g.:
```bash
    tox -e py33 -- --verbose --pdb-failures supervisor.tests.test_options
```
* 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"`:
```bash
    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()
```

Here's drilling down into one of them:

```
$ PYTHONWARNINGS="d" tox -e py33 -- -q supervisor.tests.test_options:TestProcessConfig.test_make_dispatchers_stderr_not_redirected
py33 develop-inst-nodeps: /Users/marca/dev/git-repos/supervisor
py33 runtests: commands[0] | nosetests -q supervisor.tests.test_options:TestProcessConfig.test_make_dispatchers_stderr_not_redirected
/Users/marca/dev/git-repos/supervisor/supervisor/dispatchers.py:147: ResourceWarning: unclosed file <_io.TextIOWrapper name='stdout_logfile' mode='a' encoding='UTF-8'>
  backups=backups)
/Users/marca/dev/git-repos/supervisor/supervisor/dispatchers.py:147: ResourceWarning: unclosed file <_io.TextIOWrapper name='stderr_logfile' mode='a' encoding='UTF-8'>
  backups=backups)
----------------------------------------------------------------------
Ran 1 test in 0.001s
```