Programmatically ask for permission to receive push notifications

First off, thanks for this great package.

From what I have seen, the apps using this package ask for the user permission to receive push notifications right away, on startup.

It does not seem to be the best pattern - see [this interesting article](http://techcrunch.com/2014/04/04/the-right-way-to-ask-users-for-ios-permissions/).

Is there a way we could control when to ask for the permission?

Start by tracing where the package requests push-notification permission during application startup, focusing on the iOS flow described in the issue. Determine how an application could control when that request occurs, then verify that permission is no longer requested immediately on startup and can be triggered at the intended time.

Written by the indexing model from the issue text.