Tutorial fftpack

Enhanced the fftpack example. Some open issues:

* Issue https://github.com/scipy/scipy/issues/2720 pointed out that there is an error in the dct docstring (and in the tutorial as well). I commented there; if 2720 is resolved, the tutorial needs to be updated.
* I'm not sure if copying the DCT / DST definitions from the docstring is really useful for a tutorial - any opinions on that?
* I found that the fftpack documentation does not contain dst & idst - is this on purpose or an error (although I didn't find out what keeps the docstrings from being included)?
* Not everything is included (yet); e.g. differential operators, convolutions (see mail on mailing list as I don't understand what this does)... are missing. Do you prefer PRs which are "complete" or is this step-wise approach ok for you?