Cleanup DataFrame some more

Still need to rethink how user-code looks.  Right now DataFrame is a Map
from Long (row indices) to Array[Option[Any]] - lots of overhead for hot
inner loops.
Merge branch 'master' of github.com:0xdata/h2o-dev
Merge branch 'master' of github.com:0xdata/h2o-dev
Fix makefile test order