use either pycodestyle or pep8, whatever is found.

this allows to use any python interpreter for running all subcommands
for testing.

These switches prefix all subcommands with either "python2" or
"python3". This allows to run the tests on python2 and python3 if both
python versions are installed in one system. If those arguments are not
provided, the default python enviroment is used.

tests successful on python3

this interface was added in autopep8 v1.2

this file is also included in the tests.

`os.system(cmd)` does not return the correct error codes as expected.
now replaced by `subprocess.call(cmd)` which does.

instead of `run-tests` there is a `run-tests.py` now,
that runs all the tests on execution. Tested with the latest
pythonxy on windows 7.