Update histogram_client.py

Hi,
this should do the trick, calculating the initial bin size based on [this](http://www.fmrib.ox.ac.uk/analysis/techrep/tr00mj2/tr00mj2/node24.html).

I follow your example, extracting the mean bin size for all the `data`, but I would better have an array of binsize, because in my approach it depends on the single data distribution.

Anyway,  the `auto_nbin` is not set initially to the value from the `_auto_nbin` function, any idea?

Cheers,
Mario.