Fix Remote Timing Attack vulnerability

The way the SDK compares the signed_request signature with the expected signature is vulnerable to a remote timing attack. For more information, see the following articles.

http://codahale.com/a-lesson-in-timing-attacks/
http://blog.astrumfutura.com/2010/10/nanosecond-scale-remote-timing-attacks-on-php-applications-time-to-take-them-seriously/