Add image attribution for post and page feature images

_layouts/post.html

@@ -15,6 +15,7 @@
 
 {% if page.image.feature %}<div class="image-wrap">
   <img src="{{ site.url }}/images/{{ page.image.feature }}" alt="{{ page.title }} feature image" itemprop="primaryImageOfPage">
+  {% if page.image.credit %}<span class="image-credit">Photo Credit: <a href="{{ page.image.creditlink }}">{{ page.image.credit }}</a></span>{% endif %}
 </div><!-- /.image-wrap -->{% endif %}
 
 <div id="main" role="main" itemprop="mainContentOfPage" itemscope itemtype="http://schema.org/Blog">