Support Promise API

[turnerhayes]

The latest version of requestPermission deprecates the callback interface in favor of a Promise-based interface. This library should support the Promise-based interface as well.

It looks like Safari doesn't currently support the Promise interface, but it should be possible for this library to have its requestPermission both take a callback and return a promise for backwards compatibility.

[alexgibson]

Thanks for filing this, I'll try and see to adding it soon.