Django-rest-knox

- use hmac.compare_digest instead of == for comparing hashes for more security

- drop Django 1.8 support as djangorestframework did so too in v.3.7.0
- build rest-knox on Django 1.11 and 2.0

- drop using OpenSSL in favor of urandom

- Add context to UserSerializer
- improve docs

- improved docs and readme
- login response better supporting hyperlinked fields

**Please be aware: updating to this version requires applying a database migration. All clients will need to reauthenticate.**

- Big performance fix: Introduction of token_key field to avoid having to compare a login request's token against each and every token in the database (issue 21)
- increased test coverage