add new model for photo

- add ukbench model (models/ukbench)
- ukbench model was trained by Recognition Benchmark Images (http://www.vis.uky.edu/~stewe/ukbench/)

usage:
$ th waifu2x.lua -m scale -model_dir models/ukbench -i input.jpg -o output.png

images/gen.sh

@@ -5,3 +5,4 @@ th waifu2x.lua -noise_level 2 -m noise_scale -i images/miku_small_noisy.png -o i
 th waifu2x.lua -noise_level 2 -m noise -i images/miku_noisy.png -o images/miku_noisy_waifu2x.png
 th waifu2x.lua -noise_level 2 -m noise_scale -i images/miku_CC_BY-NC_noisy.jpg -o images/miku_CC_BY-NC_noisy_waifu2x.png
 th waifu2x.lua -noise_level 2 -m noise -i images/lena.png -o images/lena_waifu2x.png
+th waifu2x.lua -m scale -model_dir models/ukbench -i images/lena.png -o images/lena_waifu2x_ukbench.png