SPARK-2143 [WEB UI] Add Spark version to UI footer

This PR adds the Spark version number to the UI footer; this is how it looks:

![screen shot 2014-11-21 at 22 58 40](https://cloud.githubusercontent.com/assets/822522/5157738/f4822094-7316-11e4-98f1-333a535fdcfa.png)

Author: Sean Owen <sowen@cloudera.com>

Closes apache#3410 from srowen/SPARK-2143 and squashes the following commits:

e9b3a7a [Sean Owen] Add Spark version to footer

core/src/main/scala/org/apache/spark/ui/UIUtils.scala

@@ -204,6 +204,11 @@ private[spark] object UIUtils extends Logging {
           </div>
           {content}
         </div>
+        <div id="footer">
+          <div class="container-fluid">
+            <p class="muted credit">Spark {org.apache.spark.SPARK_VERSION}</p>
+          </div>
+        </div>
       </body>
     </html>
   }
@@ -230,6 +235,11 @@ private[spark] object UIUtils extends Logging {
           </div>
           {content}
         </div>
+        <div id="footer">
+          <div class="container-fluid">
+            <p class="muted credit">Spark {org.apache.spark.SPARK_VERSION}</p>
+          </div>
+        </div>
       </body>
     </html>
   }