Add In-App Purchases to Native apps

Big changes incoming! :smiley_cat: 

Now that [In-App Purchases](https://tictail.com/developers/documentation/in-app-purchases/) are supported in our [REST API](https://tictail.com/developers/documentation/api-reference/) we need to allow Native Apps to charge users. So, TT.js now supports creating purchase tokens as well as opening up the payment modal, which currently is the **only** way of finalizing an In-App Purchase. :stars: 

Good to go, @JoelBesada ?

:boom: :sailboat: :ocean: 
:ocean: :ocean: :ocean: