Add support for stale-if-error Cache-Control directive

Useful for Squid and other proxies to allow stale content to be served if the server has an error:

>   The stale-if-error HTTP Cache-Control extension improves availability
>   of some kinds of cached content by allowing servers and clients to
>   instruct caches to use stale responses when certain error conditions
>   are encountered.

See [The stale-if-error HTTP Cache-Control Extension](http://tools.ietf.org/html/draft-nottingham-http-stale-if-error-01).

Also updates the plugin codebase to v1.2 from the WordPress plugin SVN repo.