API Sites: Support for varargs

[scottgonzalez]

See jquery/api.jquery.com#472

[scottgonzalez]

I apparently already did this last year in a5b5f82, but the support doesn't exist for callbacks. It's being use for http://api.jqueryui.com/jQuery.widget/#method-_super, though we might want to change the signature from _super( arg, ... ) to _super( arg... ) though perhaps having them as separate arguments makes it less confusing with rest arguments.