some improvements

[mkb79]

• you don't need to register=True when instantiate a
  :class:audible.auth. FileAuthenticator
• when you use :class:audible.auth. LoginAuthenticator it register
  as device by default
• once a device is registered and credentials are stored, you don't
  need to use LoginAuthenticator again. If you register you gain a
  adp_token and device_cert for requesting api. adp_token and
  device_cert never expires. You need no check about this. Only access
  token expires after 60 Minutes. But if you use Audible <0.3 you cant
  use access_token for request.