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