Now have working sparse with DenseColMatrix and SparseVector (ATTENTION: sparse handling is hardcoded!).
Add NPE checks.
Cleanup Sparse backprop call.
Merge branch 'master' into arno-deeplearning2

Conflicts:
	src/main/java/hex/deeplearning/DeepLearningModel.java
	src/main/java/hex/deeplearning/DeepLearningTask.java
Compute variable importances during scoring, and report on JSON.
Merge branch 'arno-deeplearning2'
Merge branch 'master' of https://github.com/0xdata/h2o