Reorganize main repository

Add nexus, probe-event, and benchmarks as submodules and cleaned up all CMake
build scripts.
Add Benchmark Suite Options to configure script
Add hook infrastructure to middleman, relates #150
Add missing `anon_send` for typed actors

Move `check_typed_input` to its own header, add `anon_send` for typed actors
and do some cleanup in `local_actor`.
Add `typed_actor::extend` to compose actor types
Add `reacts_to` alias for `replies_to::with<void>`
Build shell as submodule
Add libcaf_opencl as submodule
Merge branch 'topic/rename_interface' into develop
Merge branch 'topic/POLLPRI_workaround' into develop
Merge branch 'topic/no_POLLPRI_on_Windows' into develop
Add qtsupport with Qt5 and Qt4

Reimplement example from V0.9.4.
Build using fPIC for Qt5
Add probe-event headers to make install
Add missing hooks in BASP broker and publish()
Move opencl unit test to opencl repository.
Additions to cmakefile reguarding libcaf_opencl
Handle `optional<T>` members in `announce()`
Merge branch 'topic/announce_optionals' into develop
Merge branch 'master' into develop
Merge branch 'develop' of github.com:actor-framework/actor-framework into develop