Added tox.ini with following default testenvs run by default with `tox`:

* unit: unit tests ($> py.test tests/unit)
* self: self tests ($> py.test tests/self)
* flake8: flake8 lint ($> flake8)
* pylint: flake8 lint ($> pylint apkg)

unit and self tests are using py.test but only unit tests are run by
default when running `py.test` directly.