add tox tests, flake8; add support for python27

also:
* cfgparse_py_doctest now returns non-zero in case tests fail.
  tox requires tests to return non-zero to detect an error.
  with original implementation running cfgparse_py_doctest under tox
  always returned 0, so tox didn't detected errors

* python27 fails the tests on exception and unicode convertion;