Add framebusting to Phabricator.

Summary:
Technically we didn't have it in the first place, but should. Also
add in X-Frame-Options for double-plus-good.

Test Plan:
Created a page with Phabricator in an <iframe />, got busted out
of it. Added in the X-Frame-Options, got an empty iframe.

Differential Revision: 38
Reviewed By: tomo
Reviewers: mroch, tomo

src/aphront/response/webpage/AphrontWebpageResponse.php

@@ -34,7 +34,8 @@ class AphrontWebpageResponse extends AphrontResponse {
 
   public function getHeaders() {
     return array(
-      array('Content-Type', 'text/html; charset=UTF-8'),
+      array('Content-Type',    'text/html; charset=UTF-8'),
+      array('X-Frame-Options', 'Deny'),
     );
   }