adding the listenables prop to createStore

This adds a convenient `listenables` prop to `createStore`, which lets you easily listen to all listenables in an object (read: the return value from `createActions`). 

Here's the example added to the README. If you've previously done this:

```
var actions = Reflux.createActions(["fireBall","magicMissile"]);
```

...then instead of doing this:

```javascript
var Store = Reflux.createStore({
    init: function() {
        this.listenTo(actions.fireBall,this.fireBall);
        this.listenTo(actions.magicMissile,this.magicMissile);
    },
    fireBall: function(){
        // whoooosh!
    },
    magicMissile: function(){
        // bzzzzapp!
    }
});
```

...you can now do this:

```javascript
var Store = Reflux.createStore({
    listenables: actions,
    fireBall: function(){
        // whoooosh!
    },
    magicMissile: function(){
        // bzzzzapp!
    }
});
```

To enable this to be used with an object of stores too (hence the agnostic name `listenables`), I added support for `getDefaultData` in the listenable. That data is sent to the listening callback, or a `this.<callbackname>Default` if that exists.