Add caching

[hadley]

A resource is cacheable if all of the following are true:

1. Status code was 200, 203, 300, 301, 410 (ignore 206, partial response
   for now).
2. Method is GET or HEAD. Any other method (PUT, POST, DELETE) invalids the
   cache for that url.
3. It has one of expires, cache-control/max-age, last-modified or etag,
   and it does not have cache-control/no-cache or cache-control/no-store.
   If present, expires must be in the future.

There are two ways to retrieve from the cache:

1. If header includes Expires (absolute) or max-age directive
   Cache-Control (delta relative to date served), then can
   re-serve to future requests (if both specified, prefer max-age).
2. Possibly stale entry needs to be validated with server. Send additional
   If-Modified-Since (from Last-Modified) and If-None-Match
   (from ETag). If valid, returns 304, otherwise returns full request.

New functions need:

• parse_http_date()
• parse_cache_control()
• response_cacheable()
• response_expiry() - using both expires and cache-control headers.
  Possibly cache in request object.
• response_update() - checks if a response is still valid using by
  response_expiry() and a conditional response as necessary. Constructs
  new response if needed.
• response_cache_info() - given a request extract all caching related information
  and show in a convenient way for debugging.

Useful links:

• https://github.com/Lukasa/httpcache/blob/master/httpcache/cache.py
• https://github.com/ionrock/cachecontrol

Testing

• Heroku
• http://httpbin.org/cache/n: Sets a Cache-Control header for n seconds.
  Returns 200 unless an If-Modified-Since or If-None-Match header is provided,
  when it returns a 304.