Adding the ListenerMixinFactory

This pull request adds a `ListenerMixinFactory` shorthand, which creates the `componentDidMount` call for you. Instead of doing this:

```javascript
var myComponent = React.createClass({
  mixins: [Reflux.ListenerMixin],
  componentDidMount: function() {
    this.listenTo(myStore, this.someMethod);
  },
  // rest redacted
});
```

...you can now do this:

```javascript
var myComponent = React.createClass({
  mixins: [Reflux.ListenerMixinFactory(myStore,"someMethod"],
  // rest redacted
});
```
Using `this.someMethod` as the second argument won't work, as `this` doesn't point to the instance at this point in time. You can however of course send a callback by reference:

```javascript
var myCallback = function(){
  // clever stuff goes here
};

var myComponent = React.createClass({
  mixins: [Reflux.ListenerMixinFactory(myStore,myCallback],
  // rest redacted
});
```

So really just a small convenience, but it removes some boilerplatey stuff. And I thought it was rather cute. :)
Smart to use _.extend for this. Fortunately the ListenerMixin doesn't have a definition for `componentDidMount` (instead it has a `componentWillMount`).