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.
feat: leaner requirements, improved tox/travis support

This patch reorganizes the requirements to install only what's
necessary for using httpretty. This greatly reduces installation time
when being pulled in as a dependency for another project.

The dependencies needed for running the test suite were moved to
test-requirements.txt, and were commented to indicate what type of
testing dependency it it.

The tox file and the travis yaml have been updated to reflect the
changes in the requirements files.

Finally, .travis.yaml has leveled up. It now supports running the test
suite using pypy and python 3.3, with the added benefit that a build
will still be marked as successful even if py3.3 and pypy fail.
refactor(setup): single requirements function

Read in requirements using a refactored variant of
test_requirements().

Also, install package requirements in tox.ini.