Add support for bulk imports with COPY FROM STDIN

 * db.Prepare("COPY foo (a, b) FROM STDIN") returns a COPY IN
   statement
 * stmt.Exec(a, b) inserts new items
 * stmt.Exec() syncs the input stream, since sql.Stmt.Close() does not
   return errors from stmt.Close()
Add Oliver Tonnhofer to Contributors; update Features/Future