v2.1.3

Add support for conditional requests (#162)

This change pulls in some code copied from net/http's fs.go so that we
can support If-Match/If-None-Match requests. This will make it easy to
put a caching proxy in front of linx-server instances. Request
validation will still happen as long as the proxy can contact the
origin, so expiration and deletion will still work as expected under
normal circumstances.