pytest_00: initial py.test testing for tdi

- run_tests filters out python files under tests/ that start with "test_"
- py.test (by default) only runs those files
- tox.ini file for creation of virtualenvs for testing
- initial conversion of 3 files (9 tests)
- test coverage based on py.test

Please note:
- tox needs to find Python2.5, 2.6 and 2.7 executables, it expects these in
  '/usr/bin', but there is a PR to allow explicit specification (my
   /usr/bin/python2.7 is 2.7.3 but with the patch I get my virtualenvs from
   /opt/python/x.y.z (2.7.5, 2.6.8, 2.5.6, etc.)
- tox keeps virtualenvs (under .tox) only first time the virtualenv is
  created needs installing (in addition I have a local PyPI cache, which
  speeds up the initial setup).