Add AWS.Request.createReadStream() for HTTP data

Allows streaming of raw HTTP body data from a request into a Node.js stream object.

Sample usage:

```js
var s3 = new AWS.S3();
var params = {Bucket: 'myBucket', Key: 'myImageFile.jpg'};
var file = require('fs').createWriteStream('/path/to/file.jpg');
s3.client.getObject(params).createReadStream().pipe(file);
```