feat($rootElement): added application root element

Publish the application root element as $rootElement
so that it can be injected to other services.

test/ng/rootElementSpec.js

@@ -0,0 +1,12 @@
+'use strict';
+
+describe('$rootElement', function() {
+  it('should publish the bootstrap element into $rootElement', function() {
+    var element = jqLite('<div></div>');
+    var injector = angular.bootstrap(element);
+
+    expect(injector.get('$rootElement')[0]).toBe(element[0]);
+
+    dealoc(element);
+  });
+});