Pipe Intl messages through component hierarchy

This change pipes Intl `messages` through the component hierarchy, similar to `locales` and `formats`. A new `getIntlMessage('path.to.message')` method has also been added for convenience.

```js
var MyComponent = React.createClass({
  mixins: [ReactIntlMixin],
  render: function () {
    return <div>
      <p>{this.intlDate(new Date())}</p>
      <p>{this.intlNumber(600)}</p>
      <p>{this.intlMessage(this.getIntlMessage('product.info'), {
        product: 'Mac Mini',
        price: 2000.0015,
        deadline: 1390518044403
      })}</p>
    </div>;
  }
});

var i18n = {
  locales: ['en-US'],
  messages: {
    product: {
      info: '{product} will cost {price, number} if ordered by {deadline, date}'
    }
  }
};

React.renderComponent(
  <MyComponent locales={i18n.locales} messages={i18n.messages}/>,
  document.body
);
```

Closes #9