Added redirect event

Similar to this pull request: https://github.com/mikeal/request/pull/496

This instead emits an event which includes a function that allows altering the redirect.

Example usage:
```javascript
var search="?query=somedata";
request({
  url: "http://www.test.com"+search,
  followRedirect: true,
},function(error,response,body) {
  res.send(body);
}).on('redirect',function(redirect,updateRedirect) {
    updateRedirect(url+search);
});
```