[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
[refresh-token] add documentation on refresh token

Update README.md

[refresh-token] add curl example

fix
[refresh-token] make wording consistent, replace RENEW with REFRESH
[refresh-token] add scenario/use case to readme
[refresh-token] renamed JWT_TOKEN_REFRESH_LIMIT -> JWT_REFRESH_EXPIRATION_DELTA
[refresh-token] rename to JWT_ALLOW_REFRESH
Merge pull request #23 from doordash/refresh-token

Add refresh token feature