fix(tabs): resolves issue where md-tabs is used within ui-view by…

… removing hard requirement for `md-tabs` controller

This merely prevents an error from being fired if child directives are parsed without the expected parent controller.  This seems to be an issue with `ui-router`; however, including this fix on our end shouldn't hurt anything.

Closes #2414

src/components/tabs/js/labelTemplateDirective.js

@@ -9,9 +9,10 @@
       restrict: 'A',
       link: link,
       scope: { template: '=mdLabelTemplate' },
-      require: '^mdTabs'
+      require: '^?mdTabs'
     };
     function link (scope, element, attr, ctrl) {
+      if (!ctrl) return;
       var index = scope.$parent.$index;
       scope.$watch('template', function (html) {
         element.html(html);

src/components/tabs/js/tabDirective.js

@@ -61,7 +61,7 @@
 
   function MdTab () {
     return {
-      require: '^mdTabs',
+      require: '^?mdTabs',
       terminal: true,
       scope: {
         label:    '@',
@@ -74,6 +74,7 @@
     };
 
     function postLink (scope, element, attr, ctrl) {
+      if (!ctrl) return;
       var tabs = element.parent()[0].getElementsByTagName('md-tab'),
           index = Array.prototype.indexOf.call(tabs, element[0]),
           data = ctrl.insertTab({

src/components/tabs/js/tabItemDirective.js

@@ -6,8 +6,9 @@
       .directive('mdTabItem', MdTabItem);
 
   function MdTabItem () {
-    return { require: '^mdTabs', link: link };
+    return { require: '^?mdTabs', link: link };
     function link (scope, element, attr, ctrl) {
+      if (!ctrl) return;
       ctrl.attachRipple(scope, element);
     }
   }