replace provides_widget in prep for eliminating class-construction
replace all the class generation with AUTOLOAD

I really wanted to avoid it when I first wrote this, but it makes things much
more complicated than they really need to be.
turn plugins into objects, not classes
implement default_classes
document the changes on this branch
changelog update for this branch
Merge branch 'autoload'
make room for $VERSION
v0.200

        - plugins are no longer blessed into newly created, managed packages,
          but use AUTOLOAD to dispatch to plugins
        - plugins on the factory are stored as objects, not class names
        - the Input, Password, and Textarea plugins have been given a
          default_classes attribute