added pyFFTW patch

NOT ready for merge yet. Addresses #77.

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