* onetime (__version__): Increment to 2.0-beta2.
Start on issue #5: document the inner headers plan.

* onetime (Pad._make_fuzz): Describe a plan for inner headers in a
    comment here, explaining how it will support both sender-chosen
    noise length and future extensibility while revealing nothing
    about the version of OneTime used other than that it's >= 2.0.