Fix swf.executor: do not block when checking future.exception

In Executor.resume(), if the future is not finished, return it.
Otherwise the future is actually finished and checking future.exception
will not block.
Update tests: check that Executor.resume() does not block of first scheduled task
Bump version to 0.2.1
add an adaptor between  et simpleflow's future
AdaptorFuture will block if the task is not completed yet
simple async execution example