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Acquire lock before starting gc thread.

If 2 or more than 2 threads import at the same time, gc thread could be duplicate.
Use __new__ method to create Future instance for performance.
Refactoring async.py for performance.
Quit to import Future in __init__.py.
Make tests compatible with both 2.x and 3.x.
Quit to use __import__ to support both 2.x and 3.x. 2to3 will do.
Delete unnecessary argument check for performance.
Add optional argument `force' to Pool.kill method.
Future.receive() should raise DeadPoolError if the Pool is killed before done.
Create alias actor to async decorator.
Add option argument `block' to Pool.kill method.
Update document.
Version up to 0.1.2.
fixup! Update document.