add secure attribute option to XSRF cookie

To use a secure cookie for XSRF cookie.
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settings = dict(cookie_secret="foo",
                static_path=os.path.join(os.path.dirname(__file__), "static"),
                template_path=os.path.join(os.path.dirname(__file__), "templates"),
                xsrf_cookies=True,
                xsrf_cookie_secure=True,
                autoescape="xhtml_escape",
                debug=True,
                )
application = tornado.web.Application([('/(\d*)', MainHandler),], **settings)
application.listen(8888)