ENH: Improve cryptic error messages for invalid FFT sizes

fft([]) used to produce "error: (n>0&&n<=size(x)) failed for 1st keyword n: zrfft:n=0".  Now it says "ValueError: Invalid number of FFT data points (0) specified."

BUG: Also fix my previous error message for fftn that would raise TypeError if s were a tuple.
WIP ENH: Add "limit" argument to dijkstra
TST: remove all docstrings from test functions for stats.morestats
TST: add tests for stats.probplot, and fix minor issues and docs.

Closes gh-677 (Statistics Review).
MAINT: clean up some more code in stats/morestats.py
ENH: Limiting works
FIX: Fix old np test
MAINT: address review comments of @josef-pkt on gh-2991.

Adds an example for stats.probplot, and accepts more inputs for the dist
keyword.
STY: Minor fix
STY: Cleaner docstring, check limit
ENH: Use int shape
ENH: Faster shaping
TST: Add tests for invalid fft sizes
BUG: runtests: make -t option work again
Merge pull request #3005 from pv/runtests-path

BUG: runtests: make -t option work again
Merge pull request #2995 from Eric89GXL/signaltools-int-fix

ENH: Use int shape
Merge pull request #2992 from Eric89GXL/dijkstra-limit

ENH: Add "limit" parameter to dijkstra calculation
DOC: forward port 0.13.0 release notes from the 0.13.x branch
MAINT: forward port changes to release scripts from 0.13.x branch
DOC: stats: add a log base note to the  docstring