added pyFFTW patch

NOT ready for merge yet.

This branch is designed to speed up the correlation calculation as much as possible, using FFTs from FFTW. The public interfaces should not change, and if FFTW/pyFFTW are not installed, the old behavior will remain.

To do:
* finish implementation
* benchmarks
* unit tests
* explore ways to make FFT convolution thm computations robust to noise