adding Reflux.connect

After several iterations, I think I've arrived at a good convenience method for the common use case when you want to make a React component set its state to whatever a listenable returns.

This pull request exposes `Reflux.connect(listenable,[stateKey])`, intented to be used as a mixin much like `Reflux.listenTo`. If `stateKey` is supplied then it will do `setState({<stateKey>:data})`, otherwise just `setState(data)`.

Here's the `Reflux.listenTo` example from the README:

```javascript
var Status = React.createClass({
    mixins: [Reflux.listenTo(statusStore,"onStatusChange")],
    onStatusChange: function(status) {
        this.setState({
            currentStatus: status
        });
    },
    render: function() {
        // render specifics
    }
});
```

...which using `Reflux.connect` instead can be reduced to this:

```javascript
var Status = React.createClass({
    mixins: [Reflux.connect(statusStore,"currentStatus")],
    render: function() {
        // render specifics
    }
});
```

Isn't it beatiful? Absolutely nothing boilerplaty whatsoever! :)

I found in my `Reflux` app I could almost always use `Reflux.connect` as it was only in very rare cases that I needed to do some special logic.
To be more clear about it, comment should say that you can this.state.currentStatus in the render method. In tandem with what the documentation says above the example, since that's where the state variable (given by the key) will practically be used.