added `skip_message` utility function

this utility function allows users to skip messages manually
improved message_future API

this patch allows to pass any number of functors to `then`/`await` and
moves `receive_response` and `handle_response` functions to message_future.hpp
fixed handling of 'TIMEOUT' messages
set `util::conjunction<>::value == false`

this patch changes `conjunction` to evaluate to false if given
template parameter pack is empty
harmonized output of unit tests
improved unit test for sync send

this patch adds tests for functor-only usage
changed version to 0.6