Bug 930936 - Update Express module to v 3.4.5

https://bugzilla.mozilla.org/show_bug.cgi?id=930936
Fix Bug 940468 - Make drag/drop addings of media clips increase duration accordingly
Bug 940468 - Fix crashes caused by new code. missing return statement. r=thecount
Fix bug 944818 - Remove X-Powered-By header
2.0.18
Bug 944824 - Add X-XSS-Protection and X-Content-Type-Options headers
2.0.19