Allow running the tests from the pysaml2 root directory.

This improves previous work for running the tests via the
command "python setup.py test" from a fresh clone.

This can be very useful to integrate pysaml2 in a continuous
integration system such as Jenkins or Travis.
Add pymongo to tests_requires and remove extras_requires since it is not used
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.
Merge pull request #23 from lorenzogil/run-tests-from-root-directory

Run tests from root directory