header: add opengraph support

Fix #14

components/opengraph.php

@@ -0,0 +1,15 @@
+<?php if ( is_single() ) { ?>
+  <?php if(!has_post_thumbnail( $post->ID )) {
+    $image = get_template_directory_uri() . "/img/default-preview.png";
+  } else {
+    $image = get_the_post_thumbnail_url();
+  }
+  ?>
+	  
+  <meta property='og:url' content='<?php echo get_permalink(); ?>'/>
+  <meta property='og:type' content='article'/>
+  <meta property='og:title' content='<?php wp_title(''); ?>'/>
+  <meta property='og:site_name' content='<?php bloginfo('name'); ?>'/>
+  <meta property='og:image' content='<?php echo $image ?>'/>
+  <meta property='og:description' content='<?php the_excerpt(); ?>'/>
+<?php } ?>

header.php

@@ -11,6 +11,7 @@
 	<?php endif; ?>
 
   <?php wp_head(); ?>
+  <?php include(TEMPLATEPATH . '/components/opengraph.php'); ?>
 
 		<!-- mon icon -->
 	<link rel="shortcut icon" href="img/favicon.png">