Add modules to define a working workflow.

Workflows are defined by subclassing simpleflow.Workflow. The dataflow
relies on simpleflow.futures.Future and the helper methods:

- futures.wait()
- Workflow.map()
- Workflow.starmap()

One can check if a computation is finished in a non-blocking way with
the Future.finished property.

See tests/test_dataflow.py for examples.
Add command line tool to check a workflow in local execution mode WIP
Update activity: define Activity.__repr__
Update local.executor: add logging in .submit()
Update activity: put full module in name
Update task: add Registry class and global instance registry
Update activity: register each Activity in task.registry
Add tests.test_task with basic test for activity registration