Use scmp for constant-time string comparisons.

I applaud the inclusion of _any_ security in a demo project such as this. Because security is specifically mentioned in the [blog post](http://www.rethinkdb.com/blog/cats-of-instagram/), I thought it might be good to tightens it up just a notch to minimize vulnerability against [timing attacks](http://codahale.com/a-lesson-in-timing-attacks/).

You may want to consider updating the blog post as well.

ps. apologies for the whitespace truncation that snuck in there, my editor did that automatically. here's a koala: 🐨