Added SampleSplitSuperLearner()

Signed-off-by: Eric Polley <eric.polley@nih.gov>
Updated ReadMe to include devtools install script.
Merge pull request #8 from JackStat/master

Updated ReadMe to include devtools install script.
Added updates to CV.SuperLearner cvControl to ToDo
Merged method.AUC from ledell
Added summary.CV table for method.AUC
Updates screen.SIS, fixed error in check for method.AUC
Moved SL.bart to SuperLearnerExtra because BayesTree removed from CRAN