Add HTTP progress events

Adds 'httpUploadProgress' and 'httpDownloadProgress' events to the AWS.Request class. Usage:

Upload progress:

```js
var req = s3.putObject(params);
req.on('httpUploadProgress', function (progress) {
  console.log("Uploaded", progress.loaded, "of", progress.total, "bytes");
});
req.send();
```

Download progress:

```js
var req = s3.getObject(params);
req.on('httpDownloadProgress', function (progress) {
  console.log("Downloaded", progress.loaded, "of", progress.total, "bytes");
});
req.send();
```

Fixes #178 

Note: this feature is not supported in Node.js 0.8.x.