Use AUTH-SOURCE to retrieve encrypted token

[PuercoPop]

Instead of storing the token along with the rest of the users customization, use the auth-source library so we can store the token in an encrypted file instead.

Don't require CL-LIB anymore as we no longer need CL-ASSERT.

This is a backwards incompatible change as I prefer to not enable users to storing sensitive information in plain text, but could leave pivotal-token as customizable variable so the change is backwards compatible if you prefer.