Automatically rate limit requests made to third-party website/APIs

[SkeletalDemise]

STH should automatically rate limit requests made to third-party website/APIs so ideally #67 doesn't happen. For example http://md5.my-addr.com says Using of our online md5 database is limited in time with 15-20 requests, otherwise IP will be blocked permanently. on their site. STH should not make a request if the rate limit has been reached for a specific website/API.