Coreg-GUI: Hair

In our lab we acquire head shapes with a laser 3d scanner, which has the downside that the head shape surface corresponds to the surface of a cap worn over hair rather than the scalp surface. This PR adds the option to compensate for hair in the coregistration process. I haven't added tests yet but will if this should get accepted.

edit: for the new field see top right of the screen shot

![screen shot 2014-04-29 at 16 19 04](https://cloud.githubusercontent.com/assets/145771/2834374/0c8d1f1a-cfdc-11e3-92b5-55fedebb258b.png)