simplifying setup.py

The current setup.py uses the *trick* of parsing the requirements.txt. Although pip is very common, this is an extra requirement (implicitly) to install the package, saving just  `['numpy']`. This version work with pip (`pip install .`) but also without it (`python setup.py install`)

Moreover, pip < 1.2 had an [ugly bug](https://github.com/pypa/pip/pull/1027?source=cc) in the `parse_requirements` function, which increases the dependency pain.