tox.ini: Switch to use py.test

because it's awesome.

This might be more controversial, since test runners can be a bit of a religion... :smile: 

`py.test` can take a bunch of different arguments so I added `{posargs}` to the `tox.ini` so that options can be passed from a `tox` command to the underlying `py.test`. For example:

```
$ tox -e py27,py34 -- --tb=short --showlocals --durations=5 supervisor/tests/test_supervisorctl.py
GLOB sdist-make: /Users/marca/dev/git-repos/supervisor/setup.py
py27 inst-nodeps: /Users/marca/dev/git-repos/supervisor/.tox/dist/supervisor-4.0.0-dev.zip
py27 runtests: PYTHONHASHSEED='2805401122'
py27 runtests: commands[0] | py.test --tb=short --showlocals --durations=5 supervisor/tests/test_supervisorctl.py
============================================================================= test session starts ==============================================================================
platform darwin -- Python 2.7.6 -- py-1.4.26 -- pytest-2.6.4
collected 193 items

supervisor/tests/test_supervisorctl.py .................................................................................................................................................................................................

=========================================================================== slowest 5 test durations ===========================================================================
0.00s call     supervisor/tests/test_supervisorctl.py::fgthread_Tests::test_ctor
0.00s call     supervisor/tests/test_supervisorctl.py::test_suite
0.00s call     supervisor/tests/test_supervisorctl.py::TestDefaultControllerPlugin::test_maintail_dashf
0.00s setup    supervisor/tests/test_supervisorctl.py::ControllerTests::test__upcheck_reraises_other_xmlrpc_faults
0.00s call     supervisor/tests/test_supervisorctl.py::ControllerTests::test__upcheck
========================================================================== 193 passed in 0.31 seconds ==========================================================================
py34 inst-nodeps: /Users/marca/dev/git-repos/supervisor/.tox/dist/supervisor-4.0.0-dev.zip
py34 runtests: PYTHONHASHSEED='2805401122'
py34 runtests: commands[0] | py.test --tb=short --showlocals --durations=5 supervisor/tests/test_supervisorctl.py
============================================================================= test session starts ==============================================================================
platform darwin -- Python 3.4.0 -- py-1.4.26 -- pytest-2.6.4
collected 193 items

supervisor/tests/test_supervisorctl.py .................................................................................................................................................................................................

=========================================================================== slowest 5 test durations ===========================================================================
0.01s call     supervisor/tests/test_supervisorctl.py::TestDefaultControllerPlugin::test_maintail_dashf
0.00s call     supervisor/tests/test_supervisorctl.py::fgthread_Tests::test_ctor
0.00s call     supervisor/tests/test_supervisorctl.py::test_suite
0.00s call     supervisor/tests/test_supervisorctl.py::ControllerTests::test__upcheck
0.00s call     supervisor/tests/test_supervisorctl.py::TestDefaultControllerPlugin::test_tail_failed
========================================================================== 193 passed in 0.37 seconds ==========================================================================
___________________________________________________________________________________ summary ____________________________________________________________________________________
  py27: commands succeeded
  py34: commands succeeded
  congratulations :)
```

or

```
$ .tox/py27/bin/py.test -k test_no_config_file_exits -v
============================================================================= test session starts ==============================================================================
platform darwin -- Python 2.7.6 -- py-1.4.26 -- pytest-2.6.4 -- /Users/marca/dev/git-repos/supervisor/.tox/py27/bin/python2.7
collected 1141 items

supervisor/tests/test_options.py::ServerOptionsTests::test_no_config_file_exits PASSED

============================================================ 1140 tests deselected by '-ktest_no_config_file_exits' ============================================================
============================================================ 1 passed, 1140 deselected, 1 warnings in 0.74 seconds =============================================================
```

I often find it very useful in particular to run just a single test or a small subset of the tests. The supervisor test suite is pretty darned fast at this point so it's not a huge deal but maybe it could be later if we added more extensive integration tests.

[pytest's fixture mechanism (dependency injection)](http://pytest.org/latest/fixture.html) is pretty awesome too and can lead to powerful and succint tests.