Allow defining a cache as "shared"

[ionrock]

In the spec (https://tools.ietf.org/html/rfc7234#section-5.2.2.6) it talks about the private directive and that a shared cache MUST NOT store the response.

This starts by defining the cache as being "shared" and testing for the "shared" attribute on the cache.

Fixes: #141

[Flameeyes]

FTR, I have something along these lines in #225 as well.