Added redirect event

[Cauldrath]

Similar to this pull request: #496

This instead emits an event which includes a function that allows altering the redirect.

Example usage:

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);
});

[mikeal]

why do you need (redirect, updateRedirect), why can't you just mutate the url of the request object? if we encourage that path then it's obvious how people could also manipulate headers and other options on redirect.

[Cauldrath]

True, I moved the event to right before it calls init so everything is set up for the next request at that point and removed the additional data.