Feature requests for 0.5

[Emmanuelle Gouillart]

> - An expansion of the user guide
> - Completion of the Debian packaging
> - Object convex hulls and
> - Graph cut segmentation and/or tri-mapping

Regarding graph-cut segmentation, it's not exactly graph-cut, but I have
some code (purely Python) implementing the random walker segmentation
algorithm
(http://cns.bu.edu/~lgrady/Random_Walker_Image_Segmentation.html). It's a
very nice algorithm that overcomes some of the limitations of the
watershed, and it can label more than two phases. So I could clean up
this code a bit for skimage, I guess this could be my contribution to 0.5

Emmanuelle