Pulish output polygons as convex hull

* add new message: ModelCoefficientsArray
* publish the boundary points of the result of the clustering (with connecting the planes) as PolygonArray.
the outputs are reconstructed by Convex Hull.
* publish model coefficients to `~output_coefficients` and `~output_nonconnected_coefficients`

![screenshot from 2014-05-17 14 29 18](https://cloud.githubusercontent.com/assets/40454/3004195/6039fd12-dd84-11e3-9bab-b2380876b786.png)