Added new lab called FourierExtensions.
This time I actually added the new files...
Added images to the lab, including the plotFigs file.
Made some various minor changes to the Lab text. Created a new image and
included in the lab.
Merge pull request #382 from abefrandsen/cepstrum

Added FourierExtensions lab.
Could use another look-over. The Cepstrum section can be improved, and the 2D FFT can be expanded.