注册和声明周期
my = Polymer({
is: "proto-element",
created: function() {
this.innerHTML = 'created';
}
});
//注册之后会返回构造函数，创建实例的两种方法
var el1 = document.createElement('proto-element');
var el2 = new my();
my = Polymer({
is: "proto-el
