Use subscriber promises

[nialldonnellyfh]

Motivation

The mediatorjs library (https://github.com/ajacksified/Mediator.js) provides a useful API for dealing with asynchronous topics, however the API for dealing with requests is too difficult.

To address this, I have added the ability for the topic subscribers to return a Promise.

The publish command can then wait on the subscribers to resolve / reject instead of subscribing to unique topic IDs.

The benefit of this is a more clear separation between user code and the transport layer.

This change is not backwards compatible.