Support rest parameters

http://wiki.ecmascript.org/doku.php?id=harmony:rest_parameters
The final parameter to a function is a rest parameter if it is
prefixed by "...". FunctionExpression and FunctionDeclaration
nodes have a new "rest" property that is null if there is no
rest parameter, or contains an Identifer for the parameter.
https://developer.mozilla.org/en-US/docs/SpiderMonkey/Parser_API#Functions