[Issue 124] Adding a task and example for concurrent transfer submission

[davramov]

With Prefect, if we have a function annotated with a @task, we can use success = task_name.submit(params) to run it in the background as a non-blocking command. If we want to know the results, we can then call success.result() if we need to know the result (this is a blocking command).

This PR creates a wrapper for Globus transfers with the @task annotation in transfer_controller.py, as well as a test in examples/submit_transfer_task_async_example.py. This will be helpful in HPC flows where we want to copy data back to the beamline between steps.