Use thematic_get_wp_opt() in thematic_default_theme_layout()

Previous use of thematic_get_theme_opt() caused an infinite
loop since thematic_get_theme_opt() now fallbacks to
thematic_default_opt() which calls thematic_default_theme_layout().

Using thematic_get_wp_opt() ensures we actually check if database
option exists or not.

See #110

library/extensions/helpers.php

@@ -208,8 +208,10 @@ function thematic_available_layout_slugs() {
  */
 function thematic_default_theme_layout() {
 
+	$options = thematic_get_wp_opt( 'thematic_theme_opt' );
+
 	// use a default layout of right-sidebar if no theme option has been set
-	$thematic_default_layout = thematic_get_theme_opt( 'layout' ) ? thematic_get_theme_opt( 'layout' ) : 'right-sidebar';
+	$thematic_default_layout = isset( $options['layout'] ) ? $options['layout'] : 'right-sidebar';
 
 	/**
 	 * Filter for the default layout