Major README update, DoS hardening for bi-directional DNS setups + more

+ Added X.509 comparison table to README
+ DoS hardening for bi-directional DNS setups (where DNSChain handles
  only blockchain-based queries, and talks to another DNS server
  for oldDNS lookups, and the oldDNS server talks back to DNSChain)
+ Fixed logging of 404s for `favicon.ico` etc. for metaTLD lookups
+ Updated package.json keywords and version, but this isn't 0.2.4 yet.