Make it possible to set debug=false

[danvk]

On line 59, you call scope.parse with a single argument:

      loader.load(url, function (text) {

        onLoad(scope.parse(text));

      }, onProgress, onError);

but then later:

    function (text, debug) {
      if (typeof(debug) === 'undefined') {
        debug = true;
      }

So debug is always true. I'd prefer not to see line noise in my console. Can we either remove this (it doesn't exist in the upstream three.js repo) or optionalize it? Thanks.