Not working with 16-bit images

ssimulacra doesn't seem to work with 16-bit images

```
$ ssimulacra img1.ppm img2.ppm
OpenCV Error: Assertion failed ((lutcn == cn || lutcn == 1) && lut.total() == 256 && lut.isContinuous() && (src.depth() == CV_8U || src.depth() == CV_8S)) in LUT, file /home/trougnouf/.cache/pacaur/opencv2/src/opencv-2.4.13.6/modules/core/src/convert.cpp, line 1213
terminate called after throwing an instance of 'cv::Exception'
what(): /home/trougnouf/.cache/pacaur/opencv2/src/opencv-2.4.13.6/modules/core/src/convert.cpp:1213: error: (-215) (lutcn == cn || lutcn == 1) && lut.total() == 256 && lut.isContinuous() && (src.depth() == CV_8U || src.depth() == CV_8S) in function LUT

Aborted (core dumped)
```

as a side-note opencv > 2 doesn't seem to be supported (worth mentioning in the readme)