Improve README.md exmaple code to prevent TypeError

I don’t know how to thank you for your great component ! You component helps us a lot! 

There is a tiny flaw in README.md
```javascript
console.log('error on request %d %s %s: %j', process.domain.id, req.method, req.url, err);
```
sometimes will occur a json parse error, see this example:

```javascript
var express = require('express');
var app = express();
var fs = require('fs');

app.use(require('express-domain-middleware'));
app.use(app.router);
app.use(function errorHandler(err, req, res, next) {
  console.error('error on request %d %s %s : %j', process.domain.id, req.method, req.url,err);
  console.error(err.stack);
  res.send(500, "Something bad happened. :(");

});

app.get('/error', function(req, res){
    console.log('process.domain.id:'+process.domain.id);
    fs.open('/data/nodejs/abc.txt','r',function(er,fd){
         console.log('process.domain.id:'+process.domain.id);
         console.log('got abc.txt');
         fs.open('/data/nodejs/no-exist-file','r',function(er,fd){
                throw er;
           }
         );
    });
});


var server = app.listen(3000, function() {
    console.log('Listening on port %d', server.address().port);
});
```

it will occur an exception

```
TypeError: Converting circular structure to JSON at Object.stringify (native) at util.js:41:30 at String.replace
   (native) at Console.exports.format (util.js:35:23) at Console.warn (console.js:61:34) at Object.errorHandler [as handle]
   (/data/nodejs/testexpress/index.js:8:11) at next
   (/data/nodejs/testexpress/node_modules/express/node_modules/connect/lib/proto.js:189:17) at Domain.<anonymous>
   (/data/nodejs/testexpress/node_modules/express-domain-middleware/index.js:12:5) at Domain.EventEmitter.emit
   (events.js:95:17) at process._fatalException (node.js:249:27)
```

So I changed it into 
```javascript
console.log('error on request %d %s %s', process.domain.id, req.method, req.url);
console.log(err.stack);
```