run PCA to compute bounding box

![screenshot_from_2014-06-20 23 50 58](https://cloud.githubusercontent.com/assets/40454/3341660/842e43e2-f88a-11e3-97a2-979155f87194.png)
![screenshot_from_2014-06-20 23 51 23](https://cloud.githubusercontent.com/assets/40454/3341661/842f6c40-f88a-11e3-8543-5424f826c508.png)