Push manager. Fixes issue #2

Changes:
-Move pushRegistrationManager to ServiceWorkerRegistration (Fixes issue #2)
-PushManager --> PushRegistrationManager
-Adding PushRegistrationManager.hasPermission
".. that will notify the caller about the result of the operation"? The proposed text makes it sound like the caller MAY want to do this, while the method is a no-op unless the result gets used.