Fix to deserialize the parents properly

Hi!

I've been playing around with SpiffWorkflow for a while and found a deserialization problem in the Dictionary serializer. I've created a test so you can see the kind of scenario I'm fixing, but essentially, the task_tree tasks parents doen't get deserialized as the function to retrieve tasks based on UUID (get_tasks()) relies on the actual 'task_tree' attribute being constructed. 

I've postponed the parents deserialization to the end of deserialize_workflow where all the tasks in the task_tree had been created.