Tweak network layer, namespaces & singletons

This patch ports most changes from Boost.Actor back to libcppa. This includes:
1) a new network abstraction that is easier to maintain and not entangled with
the middleman, 2) a new default network protocol named BAP: "Binary Actor
Protocol" that replaces the previous mostly undocumented mess, 3) an all-new
and clean broker-based communication infrastructure that replaces the
peer/peer_acceptor design, 4) a reorganization of the namespaces: removed
"util" and "intrusive" and moved most of these classes to "detail", 5) a new
singleton class replacing singleton_manager, and 6) a new `actor_cast` function
replacing the clumsy `detail::raw_access` API.