Add 304 Unmodified Responses

[dwillis]

From an API user: Would you consider using ETags (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) to support more efficient access to the API? E.g. a client could send the most recent ETag it received in the If-None-Match header (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match) and get a 304 unmodified response if the resource has not been modified. Clients could then check whether any resources have been updated without straining the API too much. The ETag, could simply be a UUID or a hash of the response.