Adding aliases ADD_MONTHS and SUB_MONTHS

This is a request for consistency with oracle.

Change-Id: I463a66694a068cd773532d8f6f853a4b089b918a
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2400
Reviewed-by: Victor Bittorf <victor.bittorf@cloudera.com>
Tested-by: jenkins