add request event

sorry about the build in the diff, didn't notice what i'm `git adding`.
this is very useful if you want to log `request -> response`, when you are not in control of the request instance.

i currently have an ugly hack to achieve this:

```js
var end = req.end;
req.end = function(){
  emitter.emit('request', this);
  end.apply(this, arguments);
};
```