made underlying session.id accessible

server backed sessions have an `.id` property  ( the value of the set-cookie's header )

for whatever reason(s), this was omitted from pyramid's ISession and consequently from pyramid_beaker, which makes identifying the session a pain ( one has to look at the cookies, and hope that the current session corresponds to the incoming cookie - which may or may not be the case if a cookie.id in request has been replaced with a new session.id in response )

this patch simply gives pyramid_beaker sessions an `id` property, which proxies a lookup to the `id` attribute of the underlying Beaker object.