Please review PR21: Summed area table

[Stéfan van der Walt]

Hi all,

Two of the pull requests waiting to be merged make use of summed area
tables.  I've proposed a Cython implementation here:

https://github.com/scikits-image/scikits.image/pull/21

Please review and tell me what you think.  I'd like this to be
integrated fairly soon, if possible, because it is a bottleneck for
the histogram of gradients and template matching PRs.

Regards
Stéfan