Remove Boost from dependencies

Context-switching actors are no longer supported by the library.
Hence, Boost is no longer an optional dependency.
Fix build on GCC 4.7
Print build summary as status message
Changed version to 0.9.4
Fix segfault in `peer::dispose`, close #135
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.