Proxy settings

I've added a proxy setting to be used by your request. It uses the process.env http_proxy value.

```javascript
var proxy = process.env.HTTPS_PROXY
 || process.env.https_proxy
 || process.env.HTTP_PROXY
 || process.env.http_proxy;
...
if (proxy) {
  request = request.defaults({ proxy: proxy, timeout: 5000 });
}
```
I think its useful for those that are behind a proxy server.