Fix X-Hub-Signature Exploit

[jcampbell05]

https://chatbotsmagazine.com/how-to-kill-a-bot-with-10-http-requests-ca7eb57c2ad1#.j685u0tdj

Implement https://github.com/alexcurtis/express-x-hub into clients and consider implementing #36

[davidmann4]

Also make sure to use a constant time algorithm to compare the two values to avoid potential timing attacks.