Fix error about importing milestones, Add assignee attribute

[guyoun]

1. Because error was raised if a closed milestone at target already exists, I change state from open to all.

   {'documentation_url': 'https://developer.github.com/v3/issues/milestones/#create-a-milestone', 'message': 'Validation Failed', 'errors': [{'code': 'already_exists', 'resource': 'Milestone', 'field': 'title'}]}
   ERROR: There was a problem importing the issues.
   422 Unprocessable Entity
   DETAILS: Validation Failed
   

2. Add assignee to new_issue if issue at source has assignee.