v0.7.0

• Ensure that only 1 refresh operation will happen in parallel. If there are
  multiple things triggering the refresh, all will wait for the first one
  to finish.
• Automatically schedule a refresh operation 1 minute before the access token
  expires, if the expiry time is known.
• BC Break: If a token is known when setting up OAuth2, this now needs to be
  passed as the second argument. The old behavior still works but will emit
  a warning, and will be removed in a future release.
• 'OAuth2Token' type is now exported.