Feature/async local executor

A preview for a asynchronous local executor.

It uses the module `concurrent.futures` which is back-ported to python2.
It introduces an `AdaptorFuture` that covers the semantic gap between `simpleflow`'s future and `concurrent.futures`'s future object. The real job is forwarded to `concurrent.futures`'s `ThreadPoolExecutor`.

`ProcessPoolExecutor` seems a better choice but there's a serialization problem (seem inline comment).