Return the Store/Actions instance during create.

[tappleby]

During testing I found my self with a lot of createActions / createStores followed by the matching getters.

This PR lets you use the following syntax

let flux = new Flux();
let sessionActions = flux.createActions('session', SessionActions);
let sessionStore = flux.createStore('session', SessionStore);

sessionActions.login(true);
assert.isTrue(sessionStore.isLoggedIn());

Might also be useful if you want to inject actions into stores directly

export default class Flux extends Flummox {
  constructor() {
    super();

    let stargazersActions = this.createActions('stargazers', StargazerActions);
    let globalActions = this.createActions('global', GlobalActions);

    this.createStore('stargazers', StargazerStore, stargazersActions, globalActions);
    this.createStore('app', AppStore, globalActions);
  }
}

[acdlite]

Awesome. Thanks!