Cleanup setup.py

Remove some obsolete code and redundancies.
Externalize dependencies on cython and numpy to requirements.txt

This makes more sense, since cython and numpy are used during the
installation process (in setup.py).

Requirements can be installed via

    pip install -r requirements.txt

prior to executing ``python setup.py install``.
Convert README to reStructuredText

This allows to show a nicely formatted README on github/PyPI and text
editors with syntax highlighting.
Move setup.py to top level folder

Simplify uploading on PyPI and make it easier to include docs/tests into
source distribution.