adds support for r-maxage

r-maxage is a new directive that takes precendence over s-maxage and
max-age.

While s-maxage is honored by any shared cache, r-maxage is only taken
into account by reverse proxy caches, and thus ignored by proxy and
private caches. Setting r-maxage allows applications to ensure the
response is only cached by their reverse cache, and thus are able in
particular to purge the cache knowing that it will not remain stale in
intermediate proxy servers.
allow_reload/revalidate is not enabled by default

These comments were left over after
d2681527c53ef14ce7dd7835e596341fdf23b32c
Fix links to documentation in README

The tomayko.com/src/* URLs are broken. Working on a redirect somehow.
Remove link to mailing list from README

Mostly defunct
Fix documentation URL in gemspec
Merge pull request #63 from mislav/reload-doc

allow_reload/revalidate is not enabled by default
Merge remote-tracking branch 'fxn/master'