GUI -- Added distinct icon for nav-menu in the masthead.

Change-Id: If0ceed757cf580c80159e67af7f1b03671c78147

@@ -33,11 +33,24 @@
     box-shadow: 0 2px 8px #222;
 }
 
+#mast .nav-menu-button {
+    width: 30px;
+    height: 30px;
+    margin-left: 8px;
+    margin-bottom: 4px;
+    cursor: pointer;
+}
+.light #mast .nav-menu-button:hover {
+    background-color: #ddd;
+}
+.dark #mast .nav-menu-button:hover {
+    background-color: #777;
+}
+
 #mast img.logo {
     height: 38px;
     padding-left: 8px;
     padding-right: 8px;
-    cursor: pointer;
 }
 
 #mast img.logo:hover {

 <!-- Masthead partial HTML -->
-<img class="logo" src="data/img/onos-logo.png" ng-click="mastCtrl.toggleNav()">
+<img class="nav-menu-button" src="data/img/nav-menu.png"
+     ng-click="mastCtrl.toggleNav()"></div>
+<img class="logo" src="data/img/onos-logo.png">
 <span class="title">Open Network Operating System</span>
 <div id="mast-right"></div>

@@ -59,6 +59,6 @@
 }
 
 .dark #nav a:hover {
-    background-color: #999;
+    background-color: #777;
 }