Modernise background illumination estimation

* Function much more readable using [CyToolz syntax](http://matthewrocklin.com/blog/work/2013/11/15/Functional-Wordcount/)
* Should be faster, too.
* Add automated tests for background illumination estimates
Yes, which one works better is system dependent and a huge pain. On my machine, I have to use io, on the cluster, I believe Mahotas works better. skimage's IO is pretty broken imho and a bit of a pain point currently. Ultimately though, I do have the ability to improve skimage's version so I prefer to leave that here.