Cox Proportional Hazards: Add stratification support

Pivotal Tracker: 55945460

Other authors:
    Feng, Xixuan (Aaron) <xfeng@gopivotal.com>
    Shengwen Yang <syang@gopivotal.com>
    Rahul Iyer <riyer@gopivotal.com>

All Changes:
    - Updated Cox interface and changed function name to coxph
    - Added strata and optimizer_params parameters
    - Moved all help messaging to Python
    - Optimized the overall process (remove the intermediate step)
    - Replace plpgsql with sql for the overloaded functions
    - Implement stratified cox
    - Added Hessian to Cox Output Table
    - Added <output_table>_summary containing model details and its usage message
Cox Proportional Hazards: Add function for PH assumption test

Pivotal Tracker: 55945460

Other authors:
    Shengwen Yang <syang@gopivotal.com>
    Qian, Hai <hqian@gopivotal.com>
    Feng, Xixuan (Aaron) <xfeng@gopivotal.com>

All changes:
    - Added aggregate to compute Schoenfeld residual, element-wise correlation,
    and chi-square/p-values
    - Merged the design doc for stratified Cox including the residual
    computation part and chi-square computation
    - Changed function names
        coxph --> coxph_train
        zph --> cox_zph
Documentation: Improve formating for various modules

Changes:
    - Reformat doc in Early Stage Development modules to new template.
    - Treat online help consistently
        1. Remove from contents
        2. Remove online help section
        3. Make online help as the first step in examples
    - Remove group from SVDMF to eliminate it from docs
    - Change incorrect title in SVD documentation
Correct coxph train function to call coxph_train
Handle NULL values in input table gracefully

Pivotal Tracker: 57193570

Other authors:
        Feng, Xixuan (Aaron) <xfeng@gopivotal.com>
        Qian, Hai <hqian@gopivotal.com>
        Rahul Iyer <riyer@gopivotal.com>

All changes:
    - Implemented the NULL handling capability like R's na.oimit
    - Added num_rows_processed and num_missing_rows_skipped in the result table
    - Added more install-check cases
    - Can handle the case when all rows in the input table have NULL values
Added NULL handling in Linear Regression

* Moved all SQL code into Python
* NULL handling
Cox Proportional Hazards: Edit documentation to maintain consistency
Upgrade: Add script to upgrade to v1.3 from v1.0, v1.1, v1.2

Pivotal Tracker: 57615140

Changes:
    - Added all changelist files to upgrade to v1.3
    - Updated script to new upgrade
    - Fixed issues around whitespace between arguments
    - Release notes for v1.3
    - Updated version numbers
    - Update PGXN versioning style to accomodate requirements
Merging master into doc_master