Add support for httpDownloadProgress and httpUploadProgress events

Usage:

```javascript
var request = s3.putObject(params);
request.on('httpUploadProgress', function (progress) {
  console.log(progress.loaded + " of " + progress.total + " bytes");
});
request.send();
```

References #178
Remove LOADING state
Support httpUploadProgress,httpDownloadProgress in Node.js
Fix failing specs
Add integration tests for http progress events
Document http progress events
Fix lint warnings
Add note about Node.js support for 0.8.x
Add 'httpUploadProgress' and 'httpDownloadProgress' events to AWS.Request

Note: this feature is not supported in Node.js 0.8.x.

Example Usage for upload progress:

```javascript
var req = s3.putObject(params);
req.on('httpUploadProgress', function (progress) {
  console.log("Uploaded", progress.loaded, "of", progress.total, "bytes");
});
req.send();
```

Example usage for download progress:

```javascript
var req = s3.getObject(params);
req.on('httpDownloadProgress', function (progress) {
  console.log("Downloaded", progress.loaded, "of", progress.total, "bytes");
});
req.send();
```

Closes #178