Push manager. Fixes issue #2

Changes:
-Move pushRegistrationManager to ServiceWorkerRegistration (Fixes issue #2)
-PushManager --> PushRegistrationManager
-Adding PushRegistrationManager.hasPermission
@mvano is changing the steps to not explicitly mention |resolver| in https://github.com/w3c/push-api/pull/23. He should probably rebase his patch on top of this.