Added sample, generic.cv and generic.bagging, bug fixes

* sample function, with or without replacement

* generic.cv for cross-validation, not a wrapper of MADlib functions

* generic.bagging for bagging, not a wrapper of MADlib functions

* Improvements:
  1. db.objects, easier to delete
  2. as.db.data.frame directly supports db.data.frame

* Bug fixes:
  1. better delete of objects