Add option to force verifiable parent message transport [#2]

[johnfargo]

Reviewers: shindig.remailer, beaton,

Description:
Continuation of 28115.
Please review this at http://codereview.appspot.com/28117

Affected files:
  features/src/main/javascript/features/rpc/rpc.js
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/ProxyingLinkRewriter.java