Bugfix for IntegralImageNormalEstimation

This pull request fixes the bugs in IntegralImageNormalEstimation reported by James Stout in this [pcl-dev tread](http://www.pcl-developers.org/Bugs-in-IntegralImageNormalEstimation-distance-map-computation-td5708055.html) and also introduces a new minimum normal smoothing size parameter.

The introduced fixes seem to play well with
* COVARIANCE_MATRIX

  ![cm](https://f.cloud.github.com/assets/1241736/793457/7ed28158-ec09-11e2-9645-8bd1609e7ee0.png)

* SIMPLE_3D_GRADIENT

  ![sg](https://f.cloud.github.com/assets/1241736/793458/900623f8-ec09-11e2-8fb9-e710a529f826.png)

* AVERAGE_DEPTH_CHANGE

  ![adc](https://f.cloud.github.com/assets/1241736/793459/97694008-ec09-11e2-8a0d-f598ada71431.png)
  (the green borders is an okay thing for this method)

As for AVERAGE_3D_GRADIENT, there are weird artifacts. Unfortunately I did not have time to dig deeper into this problem.
![ag](https://f.cloud.github.com/assets/1241736/793466/031a427a-ec0a-11e2-919e-a18540e62c8c.png)