improve plot function

now has bin width and jitter options
document plot function
fix binomial output
added Lukes skew tests

but quick testing shows that they are exceptionally low power - worse
than a sign test for bias. And can't detect even extreme simulated
differences on large datasets for comparing distributions. This is not
much good. Hmmm. Maybe there's another measure of skew with more power?
update p.check

to throw an error when no p values are inside the predefined limits
chuck skew tests, introduce mean tests

these are better. Because two very different distributions will tend to
have very different means, but not necessarily different skews.
improve mean.tests