Fix cloning of nodes and guard against future problems

Fix Issue 985: http://code.google.com/p/google-caja/issues/detail?id=985

Adds a distinguished annotation for ParseTreeNode constructors to be
invoked reflectively (e.g., when cloning a tree); adds tests for
cloneability for the CSS and JS parse tree nodes; and makes a couple
of minor changes to the CSS nodes (the original source of this bug)
where their ctors were not allowing them to be successfully cloned.

[Jasvir, 2009-07-14 22:46:00]

LGTM