Fix kmeans bugs

Handle cases when clusters go empty (pick point with worse overall error
as new cluster center).
Fix bugs in large data kmeans forcing NaN centers when a given chunk had
no rows for some cluster
Use Log instead of System.out.
Add KMeansTest with seeds known to trigger empty clusters.
Add KMeanstest with large dataset (full covtype) known to trigger NaN
centers
Merge branch 'master' of github.com:0xdata/h2o-dev