Change .img-polaroid to .img-thumbnail

* Same code, but different classes was confusing
* Now .img-thumbnail just extends .thumbnail

less/scaffolding.less

@@ -77,13 +77,10 @@ img {
   border-radius: 6px;
 }
 
-// Add polaroid-esque trim
-.img-polaroid {
-  padding: 4px;
-  background-color: #fff;
-  border: 1px solid #ccc;
-  border: 1px solid rgba(0,0,0,.2);
-  .box-shadow(0 1px 3px rgba(0,0,0,.1));
+// Image thumbnails
+.img-thumbnail {
+  .thumbnail();
+  display: inline-block;
 }
 
 // Perfect circle