Anim接口改进，支持像下面这样的动画

```javascript

       var ctx = document.getElementById('Canvas').getContext('2d')

       function circle(cx,cy,r,opt){
         ctx.beginPath();
         for(var x in opt){
           ctx[x] = opt[x];
         }
         ctx.arc(cx,cy,r,0,2*Math.PI,true);
         ctx.fill();
         ctx.closePath();
       };

       circle(100,100,5,{
         fillStyle:"#999"
       });

       var o = {r:1};
       var anim = new Anim(o,{
         r:100
       },{
           easing:"swing",
           duration:5,
           frame:function(anim,fx){
             //console.log(fx.pos);
             var r = 5+fx.pos*(80-5);
             circle(100,100,r);
           }
         });
       anim.run();
```