Remove extensions from the external reference table. They should never
be part of the serialized code.

Create extensions/experimental subdirectory for experimental
extensions not officially supported by V8.

Review URL: http://codereview.chromium.org/5094003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5838 ce2b1a6d-e550-0410-aec6-3dcde31c8c00