Added function to compute analytic representation of a real signal (DSP.Util.hilbert)
Exported hilbert function
Use FFTW plan interface

By my calculations, this is about twice as fast
Tweak some tests

Fix typos and add checks for odd numbers of samples and integer
arguments
Merge pull request #16 from jfsantos/hilbert

Added hilbert to the util module (computes the analytic signal from a real signal).