Andrew's Janky FFT

I wrote this because of some issues I have with FFTW. For one, it's difficult
to change the FFT precision without going around and changing the names
of a whole bunch of function calls. It doesn't play nice with std::complex.
And the license is a bit difficult if you apply GPL special exceptions to
the code you're linking against. (update: fftw does in fact correctly handle
std::complex arrays, but the license is still an issue for me.)

Note: This is a radix-2 FFT only, meaning that your input/output size must
be a power of 2. If not, bad things are currently likely to happen.

Usage couldn't be much simpler:

FFT<float> fft(512);
float input[512];
std::complex<float> output[512];

... initialize input ...
fft.compute(output, input);

Computation is inplace except the initial permutation, which occurs out of
place to avoid destroying the input array. Also, the bit-reversal permutation
is only computed once, then stored as a table. This may or may not actually
improve the speed.

I'm able to get about 10,000 1024-point float FFTs per second on the i7-2760QM
in my laptop.

Enjoy!
-Andrew