Datetime support

[dimastbk]

Some small (in my opinion) bugs in datetime support, that I am aware of:

• support for duration in xls/xslx/xlsb ([hh]:mm:ss parsed as datetime):
• • 255:10:10.as_time() == None
• • 255:10:10.as_datetime() == datetime(1900, 1, 9, 15, 10, 10)
• fix duration in ods, when more than 24 hours (255:10:10 => PT255H10M10S.as_time() == None), see also [Feature Request] Support parsing ISO 8601 Duration strings chronotope/chrono#579
• support for 1904 date system in xlsb/xlsx
• improve some performance fixed #315: datetime parsing in xlsb, when only standard styles are present #316 (comment)

[tafia]

Thanks @dimastbk for all your work on this topic, really very appreciated.