Added (probably) the 16bit mode for hist.Luma.

Tested using a postprocessed 16bit clip and lut
Some test screenshot [top half hist.Luma, bottom half std.Lut, complete script here http://pastebin.com/TR7Fhpdu]
http://abload.de/img/testluma84ddy.png
http://abload.de/img/testlumak9cws.png