Change license to BSD 3-clause. Although I believe this software should remain free and
that modifications should also be open source, I trust social pressure to ensure that
remains true. Most of the Python science software is BSD-licensed, so it is hoped that
this switch will allow easier adoption in the future.
Re-work setup.py and cython includes to use the system pyfftw instead of a bundled
pyfftw.pxd. Also add web address for github repository.