Builders

I'm a fan of the builder pattern. It lets us hide extra classes, e.g. a white-space-compressing writer. How about this API?

```
// To compress as much as possible.
Json::StreamWriterBuilder b;
b.cs_ = Json::Writer::CommentStyle::None;
b.indentation_ = "";
Json::StreamWriter* w = b.newStreamWriter(&std::cout);
w->write(value);
delete w;
```

Objections? Suggestions?
