fixes #73, obey configured mapper on NamedStmt and other named queries

tiny slightly backwards incompatible change which probably won't affect most code using sqlx;  the embedded sql.Rows in sqlx.Rows has been changed from an sql.Rows to an *sql.Rows.  This was done to future proof it from possible copy-related issues (eg. copying mutexes or other private data would fail and break the rows), but no such issues have been reported or seen.