Zn support for SessionManager

> Just checked in (is already in current 5.0, but not identical)
>
> ===
> Name: Zinc-HTTP-SvenVanCaekenberghe.445
> Author: SvenVanCaekenberghe
> Time: 10 February 2016, 11:24:54.981219 pm
> UUID: 53f743c9-6056-4fb1-a6be-5afd1f644164
> Ancestors: Zinc-HTTP-SvenVanCaekenberghe.444
>
> Add dynamic (backwards compatible) support for Pharo 5's new SessionManager (thx Christophe) (ZnServer, ZnLogEvent)
> ===
>
> Sven