fix(linear-progress): add basic aria support

This fix adds support for aria-valuemin, aria-valuemax and
aria-valuenow. The valuemin and valuemax attributes are static and thus
added during the compile phase. The valuenow attribute is updated as
the progress bar’s value attribute changes.

This commit relates to issue #297

src/components/linearProgress/linearProgress.js

@@ -47,22 +47,29 @@ function MaterialLinearProgressDirective($timeout) {
       '<div class="bar bar1"></div>' +
       '<div class="bar bar2"></div>' +
       '</div>',
-    link: function(scope, element, attr) {
-      var bar1 = angular.element(element[0].querySelector('.bar1')),
-          bar2 = angular.element(element[0].querySelector('.bar2')),
-          container = angular.element(element[0].querySelector('.container'));
+    compile: function compile(tElement, tAttrs, transclude) {
+      tElement.attr('aria-valuemin', 0);
+      tElement.attr('aria-valuemax', 100);
 
-      attr.$observe('value', function(value) {
-        bar2.css('width', clamp(value).toString() + '%');
-      });
+      return function(scope, element, attr) {
+        var bar1 = angular.element(element[0].querySelector('.bar1')),
+            bar2 = angular.element(element[0].querySelector('.bar2')),
+            container = angular.element(element[0].querySelector('.container'));
 
-      attr.$observe('secondaryvalue', function(value) {
-        bar1.css('width', clamp(value).toString() + '%');
-      });
+        attr.$observe('value', function(value) {
+          var clamped = clamp(value);
+          element.attr('aria-valuenow', clamped)
+          bar2.css('width', clamped.toString() + '%');
+        });
 
-      $timeout(function() {
-        container.addClass('ready');
-      });
+        attr.$observe('secondaryvalue', function(value) {
+          bar1.css('width', clamp(value).toString() + '%');
+        });
+
+        $timeout(function() {
+          container.addClass('ready');
+        });
+      }
     }
   };
 }