ENH:  grow labels in parcellation

This is supposed to work basically like `grow_labels`, but creating non-overlapping labels. Each label is grown one step at a time and only into non-occupied vertices. It's WIP, I am posting now to ask if this would be a welcome addition or if it's too similar to existing functions. My use case is to create labels from MNI coordinates and I want them to be non-overlapping.

![grow-30](https://f.cloud.github.com/assets/145771/2313565/29b9e346-a30b-11e3-874b-a7056fd97c15.png)
![grow-50](https://f.cloud.github.com/assets/145771/2313568/2b54106e-a30b-11e3-9324-d7a56814882a.png)