Update pool.Task: add .process attribute
Update slot.Pool: replace BoundedSemaphore (not supported by Mac OS X) by Semaphore
Refactor tests: remove useless lambdas
Update process #6: make Process.on_sigchld() depend on Process.join()

Process.join() now disables Process.on_sigchld() by restablishing the
default handler SIG_DFL. Process.on_sigchld() calls Process.join() to
avoid calling os.waitpid() in two difference places because it could
interrupted and then resumed in the wrong place.
Update process: add ProcessOpen.is_running property
Cosmetic changes
Update process: use self.returncode in ProcessOpen.on_sigterm()
Update process: add Process.bind_signal_handlers() method
Update process: use callable()
Update process: change ProcessOpen.terminate behavior and return value to a boolean