WIP towards fixing memory leak for n_folds.
Merge branch 'master' into arno-deeplearning2

Conflicts:
	src/test/java/hex/DeepLearningProstateTest.java
	src/test/java/water/JUnitRunnerDebug.java
WIP towards fixing memory leak when cross-validation is on.
Merge branch 'master' into arno-deeplearning2

Conflicts:
	src/test/java/hex/DeepLearningProstateTest.java
Merge branch 'master' into arno-deeplearning2

Conflicts:
	src/test/java/hex/DeepLearningProstateTest.java
Use majority categorical factor for imputation of missing values.
Add enum to specify the treatment of missing values in DL.
Options: Skip or MeanImpute.
Explicitly delete the best model.
Disable newly added n_fold testing for now, still need to fix some memory leaks.
Merge branch 'master' of https://github.com/0xdata/h2o