Adds partial support for related fields to RequestFactory. Right now, getters of such fields always return null.

Review at http://gwt-code-reviews.appspot.com/741801

Review by: rjrjr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8541 8db76d5a-ed1c-0410-87a9-c151d255dfc7