adding signal rock

A decent start for a signal processing toolbox.

Documentation here:
http://soumith.github.io/torch-signal/signal/

A lot more is in progress as I need it. I thought it would be good to have faster development if other people use it as well.

FFTs, DCTs and their inverses for 1D, 2D, 3D tensors (real and complex)
Complex Cepstrum and its inverse
Hilbert Transforms
Windowing functions hann, blackman, blackman-harris
A few more matlab-like functions

An almost complete implementation of Constant-Q transforms (in signal.incomplete namespace)