simplify settings.

The simple_settings class has been moved to its own php file
and renamed simple_theme_settings.

Started moving all theme HTML generation logic back into the lib.php
file so the layout file just needs to echo the final HTML. This
idea was inspired by Julian's approach with the Elegance theme.

Possibly the bootstrap grid stuff could use this method as well.

Simplified the CSS replacement function so you no longer need to
specify the settings or the defaults, it'll automatically grab all
settings it finds for the theme.

Rearranged the lib.php file to put related stuff together, mostly
CSS at the top, HTML at the bottom.