Sobel / Prewitt Edge Detection

[Stéfan van der Walt]

Hi all,

Pieter Holtzhausen made a pull request to bring over the Sobel /
Prewitt edge detection filters from CellProfiler.  I reviewed and
merged his code, alongside with the connected components changes
suggested last week.  You can find the changesets here:

https://github.com/stefanv/scikits.image/commit/cef53c172dd3e15dc98ddbf97a908c0f67a7283b

Please review and comment--you input is highly valued!

Regards
Stéfan