MERGE support

Support for functionality similar to H2's MERGE command: http://www.h2database.com/html/grammar.html?highlight=merge&search=merge#merge

I don't know if there is a SQL standard for this and I don't care if the command is identical to H2's. I'm mostly interested in a single command to support somewhat atomic* "if pkey exists then update else insert" operation

• somewhat atomic = this doesn't have to be entirely atomic but, to the degree the data source supports the equivalent of a MERGE or JTA