[IMP] Add Format to cell with num_format_str tool
[IMP]Add a num_format_str for customze the data type
[ADD]Import datetime library
Merge pull request #77 from ClearCorp-dev/8.0_report_xls_cn

[IMP]report xls-Add a tag "num_format_str" for customize the data type in the qweb-xls templates