Provide an @Required annotation to indicate that a field must be present

```
See discussion at http://groups.google.com/group/google-
gson/browse_thread/thread/6cfdcc56742357da

During deserialization, Gson should throw a JsonParseException if it finds
that a field marked with @Required is not present.

During serialization, Gson should always write out a field marked @Required
even if it is null.
```

Original issue reported on code.google.com by `inder123` on 17 Oct 2008 at 10:31