Async scope (#158)

* added async_scope_token

* clang-format

* updated the simple_counting_scope implementation

* added missing token tests

* working on bring the scope up to date

* added state_base

* added spawn_future receiver

* added exec.prop

* added stop_when

* fix test issues with Linux and Windows

* some progress on implementing spawn_future

* fix an issue with lambda upsetting MSVC++

* added got a first cut at spawn_future

* added typenames needed by gcc

* fix the spawn_future completion behavior

* fixed the completion signatures for spawn_future

* clang format

* implemented and initial version of spawn

* renamed async_scope_token -> scope_token

* added missing associate implementation (tests are still missing)

* a first version of the async_scope components

* temporarily remove associate test

Author: dietmarkuehl