ENH: signal: spectral: Add periodogram and welch functions

I think scipy.signal should offer functions to do spectral estimation.  We have lombscargle, but no conventional DFT based spectral estimators.  I've coded up a simple (modified) periodogram and Welch's method in this pull request.  There are other methods that might be nice to include (multitaper, burg's method, etc) but I think that these make a good start.

The functions as given here don't allow cross spectra, but that could be easily remedied given what I have.  (Although I think separate functions are warranted.)  

There are no tests at present, but I'll add them soon.  I suspect there might be discussion of the signature and exposed functionality. 

I'll send a email to the discussion list, too.