[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);
```
I spotted your issue, an error raised there will be caught by your middlware, but you are not responding in this error.

```
app.get('*', function (req, res, next) {
  Router.renderRoutesToString(routes, req.originalUrl).then(function (data) {
    var body = '<!DOCTYPE html><html><head><title>' + data.title + '</title></head><body>' + data.html + '</body></html>';
		console.log(data.html);
		res.send(body);
	}).catch(function (err) {
		console.log(err);
                //you are not passing this back your request handler!
	});
});
```

compare

```
app.use(function (req, res, next) {
		if (req.originalUrl == "/favicon.ico")
			return next();

		Router.renderRoutesToString(routes, req.originalUrl).then(function (data) {
			var body = '<!DOCTYPE html><html><head><title>' + data.title + '</title></head><body>' + data.html + '</body></html>';
			res.status(data && data.httpStatus || 200).send(body);

		}).catch(function (err) {

		    if (err.httpStatus == 302 && err.location) {
		      return res.redirect(err.location);
		    }

		    if(err.httpStatus == 404){
		    	return next();
		    }
                       //note: passed back to express
			next(err);
		});
	});
```