Overall responsive img styles

`img`s are no longer responsive by default. Instead, use
`.img-responsive` as a class or mixin. Existing `.thumbnail > img` and
`.img-thumbnail` classes now make use of this as a mixin as well.

less/scaffolding.less

@@ -59,12 +59,14 @@ a:focus {
 // -------------------------
 
 img {
-  // Responsive images (ensure images don't scale beyond their parents)
+  vertical-align: middle;
+}
+
+// Responsive images (ensure images don't scale beyond their parents)
+.img-responsive {
+  display: block;
   max-width: 100%; // Part 1: Set a maximum relative to the parent
   height: auto; // Part 2: Scale the height according to the width, otherwise you get stretching
-
-  // Match vertical alignment with other comment elements
-  vertical-align: middle;
 }
 
 // Rounded corners