Update labels to include a base class

* Builds on the new button styles by requiring a `.btn-default` for the
regular, gray labels
* Updates components examples to show new stuff

dist/css/bootstrap.css

@@ -4073,7 +4073,6 @@ a.thumbnail:focus {
   text-align: center;
   white-space: nowrap;
   vertical-align: baseline;
-  background-color: #999999;
   border-radius: .25em;
 }
 
@@ -4085,6 +4084,15 @@ a.thumbnail:focus {
   background-color: #808080;
 }
 
+.label-default {
+  background-color: #999999;
+}
+
+.label-default[href]:hover,
+.label-default[href]:focus {
+  background-color: #808080;
+}
+
 .label-danger {
   background-color: #d9534f;
 }