can not add class in IE7

[luobotang]

http://stackoverflow.com/questions/531508/getattribute-cannot-return-class-in-ie7

So,

ClassList.prototype.array = function(){
  var className = this.el.getAttribute('class') || '';

shoude be updated to:

var className = this.el.className;

[luobotang]

I make a pull request: https://github.com/component/classes/pull/26