feat(alert): add WAI-ARIA markup

Closes angular-ui#1806

Author: bekos

src/alert/test/alert.spec.js

@@ -1,5 +1,4 @@
 describe('alert', function () {
-
   var scope, $compile;
   var element;
 
@@ -36,7 +35,7 @@ describe('alert', function () {
   }
 
   function findContent(index) {
-    return element.find('span').eq(index);
+    return element.find('div[ng-transclude] span').eq(index);
   }
 
   it('should generate alerts using ng-repeat', function () {

template/alert/alert.html

@@ -1,4 +1,7 @@
-<div class="alert" ng-class="{'alert-{{type || 'warning'}}': true, 'alert-dismissable': closeable}">
-    <button ng-show="closeable" type="button" class="close" ng-click="close()">&times;</button>
+<div class="alert" ng-class="{'alert-{{type || 'warning'}}': true, 'alert-dismissable': closeable}" role="alert">
+    <button ng-show="closeable" type="button" class="close" ng-click="close()">
+        <span aria-hidden="true">&times;</span>
+        <span class="sr-only">Close</span>
+    </button>
     <div ng-transclude></div>
 </div>