Feature/use python future

For issue botify-labs/simpleflow#9

It's easier than I thought. Comment please.
I assume that `concurrent.futures.Future` was not intended to be a base class as the module lacks an abstract base class. Hence it already comes with some implementation details like the `threading.Condition` used to wait until the completion of the task.

I suggest we add this missing abstract base class that comes with the `_condition` attribute.