improve context.inspect()

for #250. i think this looks clean to me:

```js
{ request: 
   { method: 'GET',
     url: '/',
     header: 
      { host: '127.0.0.1:5000',
        dnt: '1',
        cookie: '__utma=96992031.1128528549.1373563562.1392689500.1393745224.41; __utmz=96992031.1373563562.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); _ga=GA1.4.1128528549.1373563562; connect.sess=s%3Aj%3A%7B%22secret%22%3A%22EQXKogF9sdyvaCecuTt%2B13rW0T12WVGP%22%2C%22userid%22%3A%22527d85ea6381c4a471000001%22%7D.OJ%2FAMH0JnVa%2FWyK0tZnbZ1ZCmTjYwsx8s4KU7SLc%2FE0',
        connection: 'keep-alive',
        accept: 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8',
        'user-agent': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_4) AppleWebKit/537.77.4 (KHTML, like Gecko) Version/7.0.5 Safari/537.77.4',
        'accept-language': 'en-us',
        'accept-encoding': 'gzip, deflate',
        'cache-control': 'max-age=0' } },
  response: 
   { status: 404,
     string: 'Not Found',
     header: { 'x-powered-by': 'koa' } },
  app: { subdomainOffset: 2, poweredBy: true, env: 'development' },
  req: '<original node `req`>',
  res: '<original node `res`>',
  socket: '<original node `socket`>',
  originalUrl: '/' }
```