Facial Keypoints Detection (work in progress)

This PR is a work in progress. It should include the following:
- [x] dp.FacialKeypoints DataSource
- [x] nn.MultiSoftMax : https://github.com/clementfarabet/lua---nnx/pull/14
- [x] dp.Null Loss (for the test set, which has no targets)
- [x] dp.FKDKaggle Feedback (again, for the kaggle test set).
- [x] dp.KLDivergence (adapts nn.DistKLDivCriterion)
- [x] constant value baseline
- [x] dp.FacialKeypointsBaseline Feedback (for train/valid sets)
- [x] upload dataset
- [x] example/facialkeypointsdetector.lua
- [x] tutorial (Kaggle case study)