Now accepts strings for assignee.

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()
adding the tests for issue 214
meeting coding conventions
Added onetime_password to create_authorization

github.AuthenticatedUser.create_authorization has been modified to
support two-factor authentication. When two-factor authentication
is enabled create_authorization will throw an TwoFactorException.
The onetime password can be then be passed to create_authorization
on a subsequent call.
Fix #216: handle 'Link' header

Headers with capitals are returned by http.client (replacing httplib in Python 3)
Merge pull request #218 from farrd/master

Can now specify assignee as a string
Merge branch 'develop'
Integrate #220 (fix tests and pep8)
Merge branch 'pr220' into develop
Merge branch 'develop'
Publish version 1.23.0