Hash check for pages #44

Use an optional fragment for verifying the validity of a downloaded page. For example:

http://djdns.ri.hype/private/ri.json#SHA1:294897BFF...

The fragment is two-part, consisting of a hash algorithm name, and the hash value expected for the content. Only attempt to download if we don't already have a copy of the (key by non-fragment url) Document running. Abort target immediately on hash failure.