Add tests subdir layout

Hi mate,
At my workplace we have a convention of placing test files in a subdir named 'tests' next to the source file.
I couldn't find a suitable layout in the plugin, so I created one.

Out of this pull request's updated Readme:
>* **tests-subdir**: Put test files next to their related source files,
>  in a subdirectory named ``tests``. Test files are prefixed with ``test_``.
>  For example, the test file for the source file ``foo/bar.py`` is called
>  ``foo/tests/test_bar.py``.

Unit tests and updated Readme included as well.

Thanks,
Roy