Make makeUnauthenticatedRequest a public method and document it.

AWS.Service.makeUnauthenticatedRequest() allows service classes to
make unauthenticated requests to public API operations.

For example, you can get a public object in S3 with:

```javascript
var s3 = new AWS.S3();
var params = {Bucket: 'bucket', Key: 'key'};
s3.makeUnauthenticatedRequest('getObject', params, function (err, data) {
  if (err) console.log(err);
  else console.log(data.Body.toString());
});
```

Closes #206