Make `arguments` not a reserved keyword.

This patch changes the handling of `arguments` so that it is not considered a
reserved keyword. It is not listed as a reserved keyword on either MDN or the
ES5.1 specification: http://www.ecma-international.org/ecma-262/5.1/#sec-7.6.1

I verified that the following code works in IE 8, Chrome 32, and Firefox 26.

  function arguments() {
    return arguments[0] + arguments[1];
  }
  var object = {};
  object.arguments = arguments(1, 2);
  object.arguments === 3;