Version 0.9.0

• Documentation improvements.
• Unwrap support for packaged_task. packaged_task<future<R>(A...)> and packaged_task<shared_future<R>(A...)> are now allowed. get_future member function returns unwraped future<R>/shared_future<R> respectively.
• Better implementation of pc::async. Allocation of additional intermediate shared state is removed. Only one shared state is allocated in the current implementation.
• Notification on future ready. future::notify and shared_future::notify member function allow to pass custom function object to be called when future becomes ready.
• Added timed_waiter class which is intended to replace wait_for/wait_until member functions of future and shared_future. Those member functions are rarely used but prevent efficient future implementation. wait_for/wait_until are deprecates in this release.