Different Models have different preprocessing needs

[avik-pal]

Here is a gist for the preprocessing needed for Resnet50 (use the updated model from #17).

The current preprocess function handles VGG and SqueezeNet fine, but ResNet50 needs the inputs scaled between -1 and 1 and NOT -255 to 255.

What would be the proper way to handle this? (Once we add more pretrained models they will have custom preprocessing needs)

[avik-pal]

Even the densenet model seems to have some other preprocessing steps ( none of the above 2 work for it).

[darsnack]

Closing in favor of #117