Use html classes instead of templates

And now that we have classes that take an attribute dictionary we can
simplify the layout API.
Add support for specifying stylesheet

Also use more meaningful name for layout instance.
Add flat layout (make this the default)
impressme: strip XML node data

Makes the resultant HTML output a bit more readable.
layout/radial: give the first section 0 rotation

Make the title line up better with the first section. Also saves the
Radial "surprise" until a bit later.