Oracle date type compat. functions: next_day, last_day, ..

Hello

This patch is implementation of some date functions which produce better
compatibility with Oracle (On request Radim Kolar). Major changes are in
/src/backend/utils/adt/oracle_compat.c. There are new functions: next_day,
last_day, add_months and months_between.

Original Oracle doc
http://www.techonthenet.com/oracle/functions/add_months.htm
http://www.techonthenet.com/oracle/functions/last_day.htm
http://www.techonthenet.com/oracle/functions/next_day.htm
http://www.techonthenet.com/oracle/functions/months_between.htm

Best regards
Pavel Stehule