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