Create opensocial-data-base 'feature', and use this when rewriter succeeds

[awiner]

Reviewers: levik,

Message:
Tests to come before submitting, but this patch makes MutableContent
still uglier, so if anyone has good ideas for cleaning that up, it'd be
appreciated.


Please review this at http://codereview.appspot.com/20043

Affected files:
  M     features/features.txt
  A     features/opensocial-data-base/data-base.js
  A     features/opensocial-data-base/feature.xml
  M     features/opensocial-data/data.js
  M     features/opensocial-data/feature.xml
  M      
java/gadgets/src/main/java/org/apache/shindig/gadgets/render/RenderingContentRewriter.java
  M      
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/MutableContent.java
  M      
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/PipelineDataContentRewriter.java