[boost] [tasklets] new lib

boost.tasklet is a split-up from old boost.fiber. The lib allow to execute micro-tasks (tasklets) in a thread and assigns each small task with a micro-thread/fiber - so tasklets are cooperativly scheduled. The API contains classes and functions to manage and synchronize tasklets (even if the tasklets are running on different threads). boost.tasklet is an example how boost.fiber could be used for cooperative scheduling ansd will be used by the threadpool from boost.task. Oliver