feat(list): add dividers to lists

src/components/list/list.scss

@@ -175,4 +175,11 @@ md-list[dense] {
 
     @include md-line-base($md-dense-font-size);
   }
+
+}
+
+md-divider {
+  display: block;
+  border-top: 1px solid md-color($md-foreground, divider);
+  margin: 0;
 }

src/demo-app/list/list-demo.html

@@ -32,9 +32,7 @@
           <span class="demo-secondary-text">{{message.message}}</span>
         </p>
       </md-list-item>
-    </md-list>
-    
-    <md-list>
+      <md-divider></md-divider>
       <md-list-item *ngFor="#message of messages">
         <h4 md-line>{{message.from}}</h4>
         <p md-line> {{message.subject}} </p>