Adding u-net binarization

[alexdesiqueira]

Adding U-net binarization. Continues #50. Uses fastai's U-net implementation with a ResNet-34 as the encoding path. Still to do:

• [x] Add documentation to functions.
• [X] Re-check — and possibly clean — the code.
• [X] Check how to distribute the trained net, unet_butterfly.pkl (> 100 MB).