Access Rhino's native JSON.Stringify from Java

Note that Hannes has now addressed this in Rhino. So the usage simplifies to this:

import org.mozilla.javascript.NativeJSON;
// ...

Object json = NativeJSON.stringify(cx, scope, jsObject, null, null);

The org.mozilla.javascript.NativeJSON class should be public in the Rhino 1.7R4 release.