Cleaner steps for register, unregister, getRegistration

* Less verbose, no need to refer to resolver.
* Use DOMException, not DOMError.
* It's not ok to reject a promise without arguments, always provide a DOMException.
* Resolve unregister promise with PushRegistration.

For more background details see the Promises Guide:
https://github.com/w3ctag/promises-guide

This fixes issue #4.
Perhaps we could add a non-normative note indicating that even if the UA was unable to withdraw the registration with the push service, it should ensure that no further messages are delivered to the SW?