Multiple enhancements:

* Added vcov, ifelse, rowMeans, rowSums methods

* Use MADlib branch helpers_for_demo_R: pivotalr_crossprod[_sym] for
  crossprod

* Use cbind2 instead of cbind to improve performance

* lm & glm grouping results: each item in the list contains its OWN data
  wrapper, which points to the only data portion that it used for
  training

* Corrected a predict function typo

* In CD implementation, disabled p value limtation check to speed up

* In CD implementation, create intermediate table instead of view to
  speed up