Add faster implementation of FFT

And test it. This uses the same algorithm as fftpack and is as fast as the
radix 2 fftpack implementation, but uses complex numbers
directly (simplifies code).
Speedup fft_vectorized()
Implement fft_vectorized_inplace
Get rid of the N^2 algorithm
Import the inplace routine
Merge pull request #23 from certik/fft

Add faster implementation of FFT