Added properties for ".bordered". And added prefix

If you want to make your component bordered, you can add `.bordered` class name.
===
Added prefixes before `border-radius` for old browsers

css/skeleton.css

@@ -183,6 +183,8 @@ input[type="button"] {
   text-decoration: none;
   white-space: nowrap;
   background-color: transparent;
+  -webkit-border-radius: 4px;
+  -moz-border-radius: 4px;
   border-radius: 4px;
   border: 1px solid #bbb;
   cursor: pointer;
@@ -238,6 +240,8 @@ select {
   padding: 6px 10px; /* The 6px vertically centers text on FF, ignored by Webkit */
   background-color: #fff;
   border: 1px solid #D1D1D1;
+  -webkit-border-radius: 4px;
+  -moz-border-radius: 4px;
   border-radius: 4px;
   box-shadow: none;
   box-sizing: border-box; }
@@ -313,7 +317,9 @@ code {
   white-space: nowrap;
   background: #F1F1F1;
   border: 1px solid #E1E1E1;
-  border-radius: 4px; }
+  -webkit-border-radius: 4px;
+  -moz-border-radius: 4px;
+  border-radius: 4px;}
 pre > code {
   display: block;
   padding: 1rem 1.5rem;
@@ -391,6 +397,12 @@ hr {
   display: table;
   clear: both; }
 
+.bordered {
+  border: 1px solid #bbb;
+  -webkit-border-radius: 4px;
+  -moz-border-radius: 4px;
+  border-radius: 4px;
+}
 
 /* Media Queries
 –––––––––––––––––––––––––––––––––––––––––––––––––– */