Removed path_to_zentheme() since we're not conditionally including wireframes.css or block-editing.css.

Re-arranged sidebar classes so they are in no-sidebars -> two-sidebars order in the layout CSS files.

#306856: Optimize code by adding conditional includes; moved zen_preprocess_comment() to template.comment.inc.

#306856: Optimize code by adding conditional includes; moved block editing code to template.block-editing.inc.

#306856: Optimize code by adding conditional includes; refactored zen_settings_init as zen_theme_get_default_settings and moved to template.theme-registry.inc.

#306856: Optimize code by adding conditional includes; moved zen_theme() to template.theme-registry.inc.