Add gallery list for 640w size

_includes/gallery_list640w.html

@@ -0,0 +1,2 @@
+{% capture path %}gallery/{{ include.gallery }}{% endcapture %}
+{% loopdir path:path match:"*.md" sort:"ordering" %}<a href="/{{ item.fullname }}.html"><img alt="Photo: {{ item.title }}" src="{{ site.asset_prefix }}/{{ item.fullname }}~640w.jpg" title="{{ item.title }}" width="640" /></a>{% endloopdir %}