Show Master status on UI page

For standalone HA mode, A status is useful to identify the current master, already in json format too.

Author: Raymond Liu <raymond.liu@intel.com>

Closes #24 from colorant/status and squashes the following commits:

df630b3 [Raymond Liu] Show Master status on UI page

core/src/main/scala/org/apache/spark/deploy/master/ui/IndexPage.scala

@@ -86,6 +86,7 @@ private[spark] class IndexPage(parent: MasterWebUI) {
               <li><strong>Drivers:</strong>
                 {state.activeDrivers.size} Running,
                 {state.completedDrivers.size} Completed </li>
+              <li><strong>Status:</strong> {state.status}</li>
             </ul>
           </div>
         </div>