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
Let's assume srcp and dstp are of type `const uint16_t *` and `uint16_t *`, respectively. getStride always returns the stride in bytes (8 bits in a byte), but here you're indexing arrays of 16 bit integers, so you must adjust the stride to match:
```C
src_stride = src_stride / 2;
dst_stride = dst_stride / 2;
...
dstp[dst_stride * y + x] = ...;
```