2.0.0

• Fix import error when using older versions of pyjwt (refs #42)
• Make cryptography an optional dependency (see http://flask-jwt-extended.readthedocs.io/en/latest/installation.html for installation instructions)
• Add JWT_SECRET_KEY option. It will check this first, then fall back to using app.secret_key if it isn't set. Only used on symmetric signing algorithms, such as the HS* ones.
• Add JWT_PRIVATE_KEY option in place of app.secret_key for asymmetric (public/private key) signing algorithms. It now uses this option instead of app.secret_key, and is a breaking change from version 1.5.0.
• Update requirements.txt to use newest version of pyjwt