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:

```javascript
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.
It should really check if the store has the function defined so that it doesn't listen needlessly to undefined functions in the store. Not sure if it checks that here.

In the applications I've done so far, some actions in an actions object don't apply to everything a store does.