add read/write user and item obj interface in scalding mongo source.
change to use joda time.
write all users/items required fields during trainingtest split job.
These changes are related to following issues:
PDIO-42 PDIO-13 PDIO-44
Merge branch 'master' of github.com:PredictionIO/PredictionIO
update to use nscala_time lib for DateTime