[push-api] Move unregister() to PushRegistration. (#97)

This is also changing the type of returned Promise and how it is
expected to behave.
You can merge this Pull Request by running:

  git pull https://github.com/mounirlamouri/push-api unregister

Or you can view, comment on it, or merge it online at:

  https://github.com/w3c/push-api/pull/97

-- Commit Summary --

  * Move unregister() to PushRegistration.

-- File Changes --

    M index.html (61)

-- Patch Links --

https://github.com/w3c/push-api/pull/97.patch
https://github.com/w3c/push-api/pull/97.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/pull/97