Update iter_rows row_offset to min_row
Update iter_rows row_offset to min_row

This fixes issue #26.  Openpyxl removed row_offset in favor of min_row
and max_row parameters.  The rows are indexed starting at 1 rather than
0, so passing min_row=1 will retrieve all rows.

onyxfish/agate-excel,wireservice/agate-excel