Fixes bug duration variable in wait and to method

Hi. 

Although duration of _addStep must pass a number , I think must casts kicked to a number in the case of String that can be converted to a number.

exp.
```javascript
createjs.Tween.get(target).wait(1000).to({alpha:0}, “1000”).duration // => ”10001000" 
```

I think the result is that  should be 2000 because animation correctly move(2000msec)