use enum to select estimation method of NormalEstimationIntegralImage

* use enum edit to select estimation method of NormalEstimationIntegralImage
* change the default parameters for the method to covariance matrix

![screenshot_from_2014-05-23_21_49_07](https://cloud.githubusercontent.com/assets/40454/3066658/89b09888-e278-11e3-9091-a1a3e9707d61.png)