add and document extra classes and parameters

static/css/snap-gallery.css

@@ -56,6 +56,11 @@ figure figcaption {
   color: #FFF;
 }
 
+/* Extra classes for figures */
+figure.no-border .snap-wrapper {
+  box-shadow: none;
+}
+
 /** GALLERY MARKUP **/
 .snap-gallery {
   margin: 10px;