Add :act_as_shared_cache configuration option.

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 down to its interpretation of the
"private" Cache-Control directive.)
README updates for :act_as_shared_cache.
Remove redundant private decl.
Rename configuration param :act_as_shared_cache to :shared_cache.
Merge pull request #31 from rsutphin/not_always_shared_cache

Add :act_as_shared_cache configuration option