Allow running the tests inside the tests directory

This fix a regression after latest changes. In order
to run the tests from the tests directory you will
need to install the test_requires packages manually.
You can do so with a single command:

python setup.py develop easy_install pysaml2[testing]

That's why I restored the old extras_requires argument
to the setup function. Now it won't duplicate package
names but reuse the same ones as in test_requires.