Use TLS Channel-Bound Cookies

See [this blog](http://www.browserauth.net/channel-bound-cookies) from Dirk at Google:

```
This means that if such a channel-bound cookie is ever stolen off a client's machine, that
cookie won't be able to authenticate an HTTP session to the server from other machines.
This includes man-in-the-middle attackers that inject themselves into the connection between
client and server, perhaps by tricking users into clicking through certificate-mismatch warnings:
such a man-in-the-middle will have to generate its own HTTPS channel with the server, which
won't match the channel that the cookie is bound it.
```