add MERGE command

by zhai boxuan for PostgreSQL

The MERGE command is used to combine the content of two tables into one table according to the user defined matching condition. This function is useful and has been involved in other popular DBMS. However, in the latest edition of PostgreSQL, this command is not implemented yet. I am applying to participate in adding the command in PostgreSQL as a gSoC student. The whole work will include coding works all through the parser, planner and executor.