Reset POTENTIAL_HTTP_PORTS with reset

HTTPretty.reset removes all the HTTP mocks that are put in places. There
is therefore no reason to keep around potential http ports that are not
in the current list of mocks.

Therefore we should reset that set as well when we reset the mocks.
Docs: Fix a typo.

Replace "heades" by "headers".
Extract common nosetest params into setup.cfg

Arguments in setup.cfg will always be loaded as the default parameters
when running nosetest
Tell setuptools to use nose testsuite

This allows us to run the full test suite (unit and functional) by using
the python setup.py test command.
Remove tests from package but keep them in sdist

Closes: #139
Add fixtures data to sdist

Closes: #140
Include extra information into sdist package

We should be able to run tests and see licenses etc from sdist.