Postmatic aims to be friendly towards other plugins that would like to integrate with it. We'll start with a clean, easy way to add subscribers.
The only dependencies required are the Postmatic core plugin. So long as Postmatic is running you'll be all set.
Our API is part of our plugin. It wouldn't make much sense for us to package it separately since Postmatic must be active for it to have any effect. You should be able to use a test such as class_exists( 'Prompt_Api' ) to see if it's available, and make use of it if so.
You may notice that our naming prefix is Prompt and not Postmatic. Prompt was our development codename. At least it wasn't Superclassdeterministicexpertnamatrocious...