Support for serving stale content if the origin errors.
Ledge will now obey stale-if-error=1234 in cache-control headers.
Adds "stale_if_error" config option which will override the stale-if-error cache-control directive.
Add tests for stale-if-error
Merge pull request #82 from hamishforbes/stale-if-error

Support for serving stale content if the origin errors.