run PCA to compute orientation of bounding box

run PCA to compute X and Y axis of bounding box. Z axis is estimated from
the normal of the nearest plane.
You can toggle this function via `~use_pca` parameter. (default to False)

* before (X and Y are parallel to sensor coordinates)
![screenshot_from_2014-06-20 23 51 23](https://cloud.githubusercontent.com/assets/40454/3341661/842f6c40-f88a-11e3-8543-5424f826c508.png)

* after
![screenshot_from_2014-06-22 11 47 04](https://cloud.githubusercontent.com/assets/40454/3350512/b1ab27b2-f9b7-11e3-846f-25fc9281d42f.png)