XLSX I/O version 0.2.25

• fixed memory leaks in xlsxioread_sheet_next_cell_int/xlsxioread_sheet_next_cell_float/xlsxioread_sheet_next_cell_datetime (issue #54)
• added xml:space="preserve" to text cells to preserve spacing (issue #57)
• ran valgrind tests on xlsxio_xlsx2csv and xlsxio_csv2xlsx
• fixed memory leak in write_cell_data() in xlsxio_write.c (result of get_A1col was not freed)
• rewrote get_A1col() in in xlsxio_write.c to avoid issues on platforms where multiple calls to va_start()/va_end() don't work