Added kiss_fft as the default fft library. It is possible to use fftw with "cmake -DFFTWLIB=1"
Reorganize kiss_fft files (for license reasons)
Made FFTW the default as long as it is installed
Add some documentation about kissfft
Merge branch 'lingsoft-kiss_fft' into develop

Conflicts:
	aku/CMakeLists.txt