Fixes upcoming change of how rate limiting should be handled.

- Requires PHP 5.3.2 as the bare minimum from now on!
- Implements http://tools.ietf.org/html/rfc6585#section-4 as I've suggested to TMDB.

Instead of waiting and retrying after a few seconds when we reach a 500 error, when this commit gets merged the script will always wait the ideal amount of time by using the Retry-After header.