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
This cannot compile, because the srcp and dstp variables now exist only inside their respective blocks. They have to stay where they were originally.

Here is a working alternative:
```C
const uint8_t *srcp = vsapi->...;
uint8_t *dstp = ...;

if (8bit) {
   // do stuff with srcp and dstp
} else if (16bit) {
   const uint16_t *srcp16 = (const uint16_t *)srcp;
   uint16_t *dstp16 = (const uint16_t *)dstp;
   // do stuff with srcp16 and dstp16
}
```