(For Jacob) Added newBuilder(JsonClass) method to Builder classes

This PR adds a new static method to Json classes which allows the construction of a builder that's pre-filled with the values from an existing instance of the json class. I.e.,

```java
OrderJson.newBuilder(order);
```

@jburnim 
Does this return a reference (JRef?) that you can pass to `body.add` and `body._return` below?  Same question about `newBuilderFromOldMethod.param(jclass, "from")` above.  (In general, I think it is better to use the reference object created when the declaring a variable/parameter/etc., rather than retyping its name in multiple places.)