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

Because then you can easily check if the call was a no-op or not... If you have a ```pushRegistration``` instance, you can just call ```pushRegistration.unregister().then(function(b) {...});``` and use ```b``` to know whether the unregistration happened instead of doing ```navigator.push.getRegistration().then(function(r) { return r ? r.unregister() : null; }).then(function() { ... });```.

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