Can now specify assignee as a string

Issue #214

When calling pygithub for calls associated with assignees you can now simply pass a string instead of a NamedUser object. This applies to the following methods:
- issue.edit()
- repo.create_issue()
- repo.get_issues()
- repo.has_in_assignees()
- repo.has_in_collaborators()
- repo.remove_from_collaborators()
- repo.add_to_collaborators()