Add option to use flash for storage, instead of stash.

This is useful if you use the Post-Redirect-Get pattern
and want to display validation errors on the *next* request
instead of the current request.

Note that this setting will apply to all validators.
Add option to control the validator class name with a callback.

I'm not sure if this is a good idea in general, but it let's me
do neat things like automatically map to validators.  For example,
I may want My::Controller::action() to always validate with a class
named My::Validator::Action.
One more test to cover exceptions.

Also added some commentary for future readers.