[added] Router.renderRoutesToString

This PR allows the path to be given to <Routes> as a prop.

```javascript
<Routes initialPath="/some/path"> ... </Routes>
```
This only works server side. This patch also stops the URLStore and RoutesStore being started as componentWillMount is called but componentWillUnmount isn't call on server side. This would lead to a memory leak and potential async issue when multiple pages are being rendered at once.

When rendering on server side you should always make a new instance of `<Routes>` so if multiple pages are being rendered they don't change the routes in another page.

```javascript
var App = React.createClass({
  render: function(){
    return <Routes initialPath={this.props.initialPath} location="history"> ... </Routes>
  }
});

//server side
React.renderComponentToString(<App initialPath="/some/path" />);

//client side
React.renderCompoent(<App />, domelement);
```
Unfortunately, `\/script` gets replaced with `/script` by Webpack's Uglify plug-in, which completely breaks the page.

May I recommend

    var initialDataScript = '<script type="text/javascript">window.__ReactRouter_initialData=' + JSON.stringify(initialData) + ';<' + String.fromCharCode(47) + 'script>';

instead?