Refactor data pipelining execution

[awiner]

Reviewers: shindig.remailer,

Description:
Refactors data pipeling execution into a PipelineExecutor class, and use
that class from both PipelineDataContentRewriter and ProxyRenderer.
This gets ProxyRenderer support for multi-batch pipelines, and makes
PipelineDataContentRewriter and ProxyRenderer appreciably simpler.
Please review this at http://codereview.appspot.com/33067

Affected files:
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/preload/PipelineExecutor.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/render/ProxyRenderer.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/rewrite/PipelineDataContentRewriter.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/preload/PipelineExecutorTest.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/render/ProxyRendererTest.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/rewrite/PipelineDataContentRewriterTest.java