auto-invoke optgroups in CGI::FormBuilder

For option lists with three-item entries (value, label,
category), turn on the optgroups option in CGI::FormBuilder
for that field.
Documentation (user and code) tweaks

Just some clarifications, improved pod, etc about the
optgroups feature and behavior.
Merge pull request #33 from msouth/feature/optgroups

Feature/optgroups
Pod, README, VERSION, Changes for relase 0.91

- bumped VERSION to 0.91
- updated README with pod2readme
- added changes to Changes
Added backward compat option for url gen

Added backward compatibility option for url generating.  Didn't
want this to be released without a simple way for people to
go back to the old way if the new url stuff breaks things
for them.