Add :act_as_shared_cache configuration option

Adds a new configuration option, `:act_as_shared_cache`. This option, which defaults to true, indicates whether the middleware should behave like a shared cache according to the description in RFC 2616. (Basically, this comes own to its interpretation of the "private" Cache-Control directive.)

This PR addresses #29.

Includes some Cache-Control-related test updates — I noticed that a couple of the specs didn't break when I had broken something, so I fixed them.

I look forward to your feedback.