Stop using "1" as placeholder for cache-control.

max-stale being present without a value isn't the same as having
value=1, so this needs to change.
Pay attention to max-stale values in requests' cache-control headers.
Remove distinction between returnable stale responses and fresh ones
Merge pull request #21 from gregjones/max-stale

Pay attention to max-stale in requests' cache-control header