Add getInititalData() to Store

[SimonDegraeve]

Add the getInitialData() method to Store as mentioned in #44.
This method is called when an optional third argument is provided to listenTo in a React Component using the ListenerMixin.

Example:

var aStore = Reflux.createStore({
    init: function() {
        this.listenTo(anAction, function() {
            this.trigger('not initial data anymore');
        });
    }
    getInitialData: function() {
        return 'the initial data'; // Can also return a Promise
    }
});

var Component = React.createClass({
    mixins: [Reflux.ListenerMixin],
    getInitialState: function() {
        return {};
    },
    componentDidMount: function() {
        this.listenTo(
            aStore, 
            changeCallback, 
            changeCallback // This callback is optional, execute getInitialData from aStore
        );
    },
    changeCallback: function(data) {
        console.log(data);
        this.setState({
            data: data
        });
    }
});

TODO:

• write the tests

[spoike]

Since this part of the listenTo method (L24-34) is exactly the same in both createStore and ListenerMixin it'd be nice to have this extracted to a function in src/utils.js (in order to avoid double maintenance).

Something like _.handleInitialData(listenable, initialCallback).

[SimonDegraeve]

Indeed. I will update it when I go home. Any other suggestions?

[spoike]

I'll accept this PR once there are tests for it. :-)