Don't remove workdir by default if user specified

[jkeifer]

When a workdir is set for a Task the workdir will no longer be removed by default. That is, the save_workdir argument to Task constructor now defaults to None, and if left as None the default behavior is now conditional on whether or not a workdir is specified:

• If workdir is None, a temp directory will be created and save_workdir will default to False (remove working directory).
• If a workdir is specified, then save_workdir will default to True (keep working directory).

In either case, an explicit True or False value for save_workdir will take precedence.