Add support for desired class distribution when balance_classes is enabled.
HEX-1918: Specify per-class sampling factors for DL.
Also print sampling factors for all classes during stratified sampling.
HEX-1918: Add R documentation for class_sampling_factors.
HEX-1918: Update R Vignette for DL for new class_sampling_factors parameter.
Merge branch 'master' into arno-deeplearning2
Don't train with too many rows if train_samples_per_iteration is
more than the number of rows in the training data.
Improve assertion for number of training rows.

Conflicts:
	src/test/java/hex/DeepLearningProstateTest.java
Revert local changes.
Merge branch 'master' of https://github.com/0xdata/h2o