Specify empty authentication_classes #27
[refresh-token] add refresh token endpoint

typo

[refresh-token] Refactor renew token stuff, add jwt_get_user_id_from_payload_handler for custom user_id format

[refresh-token] add tests for refresh token feature

some more api checks
Fix some errors with Python2.6 and Python3

For Python2.6: fix non-existing total_seconds by converting datetime
delta to seconds.

For Python3: fix "datetime is not JSON serializable" issue by converting
the delta datetime to unix timestamp first.
[refresh-token] rewrite refresh token tests without utcnow patching

[refresh-token] cleanup
[refresh-token] clear authentication_classes for refresh token view