fix(calendar): Make calendar directive IDs unique

Prefix all calendar id attributes with the directive scope $id. This
makes it possible to include multiple calendar instances on the same
page.

src/components/calendar/calendar.js

@@ -30,6 +30,9 @@
   // TODO(jelbourn): make aria-live element visibly hidden (but still present on the page).
 
   function calendarDirective() {
+    // Generate a unique ID for each instance of the directive.
+    var directiveId = 0;
+
     return {
       template:
         '<div>' +
@@ -48,6 +51,7 @@
       link: function(scope, element, attrs, controllers) {
         var ngModelCtrl = controllers[0];
         var mdCalendarCtrl = controllers[1];
+        mdCalendarCtrl.directiveId = directiveId++;
         mdCalendarCtrl.configureNgModel(ngModelCtrl);
       }
     };