• Added worksheet_set_h_pagebreaks() and worksheet_set_v_pagebreaks()
  functions to define worksheet page breaks.
• Added LXW_FOREACH_WORKSHEET() macro to allow iteration over all the
  worksheets in a workbook.
• Added worksheet_set_print_scale() function to set the scale factor for
  the printed page.
• Added worksheet_set_start_page() function to set the start page number
  when printing.