Add temporary arg to dbListTables and dbWriteTable to control temporary table behavior

The new logical argument 'temporary' was added to dbListTables and
dbWriteTable. It defaults to FALSE for both functions. When TRUE, temporary tables
are included in the result for dbListTables. For dbWriteTable, a
temporary table is created when TRUE.

NOTE: R CMD check reports doc warnings. I'm unclear how we go about updating the docs and would very much like to add a make target to build/Makefile so this can be done quickly.

ping @hadley