Upgrade the caching policy to obey all appropriate HTTP headers (both request and response)

The current implementation is very dumb - it always caches the content and it always returns the cached data if it has it. We should optionally allow configuring the policy to obey cache headers as described here:

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

For example, the Cache-Control directives.