Minor clean up for the code
Fixed a bug in type cast

x[,] expands an array if x is a single array. This caused type-casts
expands an array when the db.obj has only one column.

Need to test more on this.
Update Readme.md file to reflect the moving of repository
Fixed typos
C++ framework & Coordinate descent for elastic net

* Added C++ framework

* Added the coordinate descent optimizer for elastic net
  - only in PivotalR
  - for small feature numbers
  - might be faster
Greatly improved the performance of .unique.string

The previous .unique.string was so slow that it made many operations
slower that they should have been.
Added db.q function for executing SQL from R
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
Improved performance of elnet cd method for binomial
Avoid using Reduce to improve performance
Improved the performance of elnet cd

Almost the same speed as fista on GPDB
Corrected a bug for dealing with array columns
Remove pivotalr prefix in crossprod
Fixed a bug in formula analysis, allowing x[1:99] syntax
Minor corrections that makes array work

Seems that everything is working now
Allow grouping to use expressions
Allow grouping to use expressions
Improvemet for ops

* Added == and != for logical values

* keep .is.agg values for simple operations (i.e. db.obj v.s. a
primitive data type)
Bug fixes:

* Fixed is.agg problem for ops
* Fixed delete bug for elnet result
* Forgot to return the result in generic.cv
Minor bug fixes
Minor fix