Mobile apps marketing promotion tracking

I am looking for a solution based on the following high level requirements: I have a mobile app on iOS and Android which is a subscription service for a news magazine. We advertise online and in other mobile apps. We want to be able to track a user from the point the user clicks our advert, then downloads the app and then purchases a subscription through the app. The solution needs to support both, Android and iOS platforms. We advertise in other apps as well as online. So the solution needs to also be able to track user clicking an advert online that leads them to app download from Android market or iTunes store, then synching app with phone and then subscribing through the phone. Please note that I am **not** looking for something that purely measures conversions i.e. click through to download as we already do this. I am interested in the next step i.e. from click through to subscribe.