docs(rating): add aria-labelledby

- Add usage of `aria-labelledby` to aid in demonstrating accessibility

Closes angular-ui#4246
Closes angular-ui#4348

src/rating/docs/demo.html

@@ -1,6 +1,6 @@
 <div ng-controller="RatingDemoCtrl">
     <h4>Default</h4>
-    <rating ng-model="rate" max="max" readonly="isReadonly" on-hover="hoveringOver(value)" on-leave="overStar = null" titles="['one','two','three']" ></rating>
+    <rating ng-model="rate" max="max" readonly="isReadonly" on-hover="hoveringOver(value)" on-leave="overStar = null" titles="['one','two','three']" aria-labelledby="default-rating"></rating>
     <span class="label" ng-class="{'label-warning': percent<30, 'label-info': percent>=30 && percent<70, 'label-success': percent>=70}" ng-show="overStar && !isReadonly">{{percent}}%</span>
 
     <pre style="margin:15px 0;">Rate: <b>{{rate}}</b> - Readonly is: <i>{{isReadonly}}</i> - Hovering over: <b>{{overStar || "none"}}</b></pre>
@@ -10,6 +10,6 @@ <h4>Default</h4>
     <hr />
 
     <h4>Custom icons</h4>
-    <div ng-init="x = 5"><rating ng-model="x" max="15" state-on="'glyphicon-ok-sign'" state-off="'glyphicon-ok-circle'"></rating> <b>(<i>Rate:</i> {{x}})</b></div>
-    <div ng-init="y = 2"><rating ng-model="y" rating-states="ratingStates"></rating> <b>(<i>Rate:</i> {{y}})</b></div>
+    <div ng-init="x = 5"><rating ng-model="x" max="15" state-on="'glyphicon-ok-sign'" state-off="'glyphicon-ok-circle'" aria-labelledby="custom-icons-1"></rating> <b>(<i>Rate:</i> {{x}})</b></div>
+    <div ng-init="y = 2"><rating ng-model="y" rating-states="ratingStates" aria-labelledby="custom-icons-2"></rating> <b>(<i>Rate:</i> {{y}})</b></div>
 </div>