Adding support for iOS notification service

[mayurdhurpate]

As per the Mixpanel docs, to get detailed insights and send rich media in notifications, we need to integrate Mixpanel Notification Swift SDK to the app.

The steps as mentioned here requires changing Podfile and NotificationService.swift along with some X Code changes. Can this be added as a part of the library too? I haven't worked much on iOS but if guided, I can try submitting a PR too for the same.

[yelkamel]

I need it too :)