Cleanup row-wise apply some

Proper error for missing arg e.g. "ddply(data,,fun)"
Implement row-wise for mean
Throw unimpl (was returning null by mistake) for missing row-wise
Tests for above
Merge branch 'master' of github.com:0xdata/h2o
Fix test
Merge branch 'master' of github.com:0xdata/h2o