0.8-beta: async and other maintanance

• defer coroutine builder is renamed to async.
• lazyDefer is deprecated, async has an optional start parameter instead.
• LazyDeferred interface is deprecated, lazy start functionality is integrated into Job interface.
• launch has an optional start parameter for lazily started coroutines.
• Job.start and Job.isCompleted are introduced.
• Deferred.isCompletedExceptionally and Deferred.isCancelled are introduced.
• Job.getInactiveCancellationException is renamed to getCompletionException.
• Job.join is now a member function.
• Internal JobSupport state machine is enhanced to support new (not-started-yet) state.
  So, lazy coroutines do not need a separate state variable to track their started/not-started (new/active) status.
• Exception transparency in Job.cancel (original cause is rethrown).
• Clarified possible states for Job/CancellableContinuation/Deferred in docs.
• Example on async-style functions and links to API reference site from coroutines guide.