First pass at implementing ValueProxy support.
Resolve issue 5522, issue 5373, issue 5559.
http://code.google.com/p/google-web-toolkit/wiki/RequestFactory_2_1_1
Patch by: bobv
Review by: rchandia,rjrjr

Review at http://gwt-code-reviews.appspot.com/1108802


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9252 8db76d5a-ed1c-0410-87a9-c151d255dfc7