added ExcelSheet::__construct() / added default date format when writing dates

added ExcelSheet::__construct(ExcelBook $book, $name);

added default date format on write()
$sheet->write(1, 1, $timestamp, null, ExcelFormat::AS_DATE)

Before change:
$sheet->isDate(1, 1); --> false (format was missing and therefore is a normal number)

After change:
$sheet->isDate(1, 1); --> true