add some comments to PsiturkConfig

Might not be clear why things work the way they do otherwise...
Revert "Switch dependency to gnureadline to avoid the whole module name issue"

This reverts commit aa772aecf2dc27ea8fbd88fa5929d3823a08ace3.

Conflicts:
	setup.py
Add warning if user is using libedit readline

Switching to gnureadline was too good to be true. Underlying cmd/cmd2
classes import regular old readline, so without going in and editing
those and shipping our own version with psiturk we're stuck using a
module with the name readline.