Added contrastive loss layer, associated tests, and a siamese network example using shared weights and the contrastive loss.
[model zoo] ignore models -- only for reference or zoo
[example] resurrect imagenet training scripts
[example] groom siamese notebook
[docs] order ipython notebooks
Merge pull request #959 from nickcarlevaris/contrastive_loss

  Add contrastive loss layer, tests, and a siamese network example