Fix bugfix to segfault in `peer::dispose`

This patch is a followup for d78fe26 that didn't quite fix the bug.
Make `ref_counted` compatible to boost-style ptrs
`optional_variant<...>` => `variant<none_t, ...>`

This patch removes the weird `optional_variant` class from libcppa in favor of
a more general `variant` implementation that simply uses a `none_t` to indicate
a non-existing value.
Remove Qt Creator files
Refactor {object => uniform_value}
New terminology, e.g., `any_tuple` => `message`

This patch renames several classes to give programmers without functional
background a better intuition about the purpose of a particular class. For
example, `partial_function` is not a concept familiar to all C++ developers and
has been renamed to `message_handler`. Although the former is a better and more
general name in theory, the latter is a better description for what the class
is actually used for in the library.
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.
Replace "typedef" with "using"
Remove implicit type cast in `typed_actor`

This patch removes the `static_cast` from operators -> and * in `typed_actor`.
Those casts are not safe, because the handle can point to an unrelated type
that only happens to support given interface. Fixes #136.
Check for _WIN32 to detect Windows hosts

This patch follows the suggestion by nubcase from #137.