Allow running specific tests from tox

A little bit of reworking of the `tox` setup so that one can run a subset of the tests -- e.g.:

```bash
$ tox -e py26 -- supervisor.tests.test_options
py26 develop-inst-nodeps: /Users/marca/dev/git-repos/supervisor
py26 runtests: commands[0] | nosetests supervisor.tests.test_options
.............................................................................................
----------------------------------------------------------------------
Ran 93 tests in 0.516s

OK
___________________________________________________________________________________ summary ____________________________________________________________________________________
  py26: commands succeeded
  congratulations :)
```