Alien::FFTW3; parametric rank; normalized inverses

Hey, Dima,

Here's the pull request I promised.  The code should work the same as before but now delegates finding the libraries to the Alien::FFTW3 module, which should permit generalizing to other platforms in a more, well, general way.  It also includes both regular and normalized inverses, and parametric rank.  The original calls are all unchanged.

All the best,
Craig