Let the full-width page template override the current theme layout

Page template chosen by the user trumps any other layout setting.
Check page template first in thematic_body_classes() to avoid a double
layout body class

see #55
Use layout theme option as default layout if it has been set

Only return `right-sidebar` as default when first installing the theme.
The default layout will be pulled from theme options after that. Fallback
to layout set in theme options if invalid layout is returned from filter
`thematic_default_theme_layout`.

see #55