[boost] [threadpool] new version - with rescheduling of current task

Hi, I've uploaded a new version of boost.threadpool. It contains a free function this_task_reschedule_until() which lets the current task reschedule until a condition (passed to the fucntion) becomes true. This allows to synchronize with other async events without blocking the worker-thread (thx to Vicente). I've also added two function in order to wait for multiple tasks (it uses Anthonies future lib wait_for_any()/wait_for_all() ). Oliver