cache length var

increases speed of _forEach loop (http://jsperf.com/caching-array-length)

![image](https://cloud.githubusercontent.com/assets/754158/3403343/aed0ca68-fd65-11e3-94a7-2a02e59d4ff6.png)

![image](https://cloud.githubusercontent.com/assets/754158/3403338/a515ad72-fd65-11e3-94b8-8cfa62595b12.png)