New PIESNO example and small corrections

The PIESNO method was merged in Dipy without an example. Here is an example for the Dipy Gallery. 

At the same time, the PIESNO method was corrected by @samuelstjean to better match the Koay et al 2009 paper. Other examples have also been uniformized. 
Best way to know is to try it, the method is fully automatic so it will pick what it can. Sometimes it will find the skull and being a region of noise, but not the ventricules from what I have seen.

Also, if there is no background, it might just not find anyhting qualifying as background/noise either.