Make Listener/Publisher methods classes

[devinivy]

Listener and Publisher methods cannot act as proper mixins, as they rely on certain contextual variables– for example, publishers have a this.emitter and listeners have this.subscriptions.

Suggestions are,

1. To privatize contextual variables (see Use of 'subscriptions' variable refluxjs#442).
2. Extend the publisher directly from the event-emitter class.
3. To turn the mixin-style methods into constructors with prototypes.
4. In turn allow createStore() and createAction() to be used as classes (i.e. new Reflux.Store(), new Reflux.Action()) (see Support ES6/TypeScript classes in Stores and Action types refluxjs#444).