Add `If-None-Match` and `If-Modified-Since` headers as appropriate to the feed fetcher's requests. On well-configured web servers, this should short-circuit the request and reduce the amount of bandwidth and processing that is necessary.
A new table was added to the FeedDb to hold one etag per domain.
If-Modified-Since semantics are based on the creation date for the feed database, which should serve as a cutoff date for the earliest update we can have received.
This completes the changes for Issue #136.
