Make assignee optional in list_tasks

[mjmare]

get_project_tasks does not return full records for tasks (strangely get_subtasks does). This mirrors the Asana API. Of course I could issue a get_task for each tasks, but that is slow when querying many tasks.

If, instead, one wants to get all tasks in 1 go and filter for project in python code one would use list_tasks. Unfortunately it requires a value for assignee, which is not desired.

So please make assignee an optional argument. I think the Asana API allows for this.

TIA