allow nan as mssing
Update utils.R

add parameter missing
Update xgb.cv.R

add parameter missing
Update xgboost.R

add parameter missing
Update predict.xgb.Booster.R

add parameter missing
Update README.md
Remove tools folder
Added OS X OpenMP instructions
Merge pull request #111 from tfgit/master

OS X OpenMP support instructions
Fixed README
Merge pull request #112 from tfgit/master

Fixed README