API/api doc change for register/unregister device

[cheungpat]

A developer is confused which API to call for registering/unregistering device:

• registerRemoteNotificationDeviceToken:completionHandler:
  • should add a method registerDeviceWithDeviceToken:completionHandler: to be consistent with other APIs
  • mention that this method is for registering remote notification (i.e. push notification)
  • mention that app should call unregisterDeviceCompletionHandler: to unregister device
• registerDeviceCompletionHandler:
  • mention that this method is for registering the device if user denied push notification permission / to register device for pubsub
  • mention that app should call unregisterDeviceCompletionHandler: to unregister device
• unregisterDevice
  • should be deprecated, call unregisterDeviceCompletionHandler: with nil parameter
• unregisterDeviceCompletionHandler:
  • mention that this method should be called to unregister device

[b123400]

Just noticed we already have deregistering device when user logouts, it's strange that we need to call it manually https://github.com/SkygearIO/skygear-SDK-iOS/blob/ac8532b087a156902e56583aa64a371b02ae2a13/Pod/Classes/SKYContainer.m#L438

[cheungpat]

@b123400 Interesting observation. I am going to merge the pull request regardless.