[Caja] Allow features other than opensocial to tame their own libraries

[jasvir]

Reviewers: Paul Lindner,
Description:
New feature library developers need the ability to tame their own
libraries.  This change exposes functions for a library to register a
function which is run after caja is loaded to tame and expose their
library to a cajoled gadget.

Please review this at http://codereview.appspot.com/104067

Affected files:
  M     features/src/main/javascript/features/caja/taming.js
  M      
java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/CajaContentRewriter.java