[TIMOB-17640] Adding and updating UserNotification methods

https://jira.appcelerator.org/browse/TIMOB-17640

- `remoteNotificationsEnabled` calls `isRegisteredForRemoteNotifications` on iOS8 and up
- `enabledRemoteNotificationTypes` calls `currentUserNotificationSettings` on iOS8 and up
- method name change `registerForLocalNotifications` ---> `registerUserNotificationSettings`
- `registerUserNotificationSettings` takes an array of types
- new property ---> `currentUserNotificationSettings`
- notification constants in Ti.App.iOS are prefixed with USER_
	- eg. `NOTIFICATION_TYPE_BADGE` ---> `USER_NOTIFICATION_TYPE_BADGE`
	- To avoid confusion with similarly named constants in Network.
- TiAppiOSNotificationCategoryProxy ---> properties are creation time only
	- `identifier` is readable after creation