- Changed WeakReferenceCallback to take a Persistent<Value> instead of
  a Persistent<Object>.
- Removed Message::GetUnderline and Message::GetScriptData.
- Added Value::IsDate, Date::Cast and Date::Value.


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@564 ce2b1a6d-e550-0410-aec6-3dcde31c8c00