Initial implementation of printing the AST as a JSON string. This

Initial implementation of printing the AST as a JSON string.  This
implementation is not yet complete (it prints only the node name for
some AST constructs) and does not serialize everything needed to
reconstitute the AST.

It is motivated by a desire to prototype source-to-source
transformations in JavaScript itself (or anything else that can grok
JSON), but it should have other uses too.

Feedback is welcome.
Review URL: http://codereview.chromium.org/131101

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3051 ce2b1a6d-e550-0410-aec6-3dcde31c8c00