Implement osx:Variable element, w/some refactoring

[awiner, 2009-07-14 22:46:46]

This patch implements <osx:Variable> for both data pipelining and templating, as
discussed on the opensocial spec group.

This required a bit of refactoring/cleanup in PipelinedData.  Most of the
interesting code in this patch is:
  - PipelinedData
  - PipelinedDataPreloader
  - VariableTagHandler

The patch also moves most of the tags into a templates.tags subpackage, which
accounts for most of the diffs.

[levik, 2009-07-15 15:52:42]

This implementation seems to define the variable for the entire invocation scope
of a template (rather than scoped to the parent element of the <osx:Variable>
tag as was initially proposed on the discussion thread.

http://codereview.appspot.com/91118/diff/1/39
File
java/gadgets/src/main/java/org/apache/shindig/gadgets/templates/tags/VariableTagHandler.java
(right):

http://codereview.appspot.com/91118/diff/1/39#newcode48
Line 48: processor.getTemplateContext().getMy().put(key, value);
I think putting the variable into Cur is more appropriate than My, since My is
used to access XML attributes and Cur is used to access runtime data.