Feature/async local executor

[darkjh]

A preview for an 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 (see inline comment).

[darkjh]

Also simpleflow.futures.wait method should not raise ExecutionBlocked exception.