[ADD] l10n_sg: Chart of accounts and chart of taxes for Singapore

(Rebase of PR #1057)
[FIX] purchase_requisition: use fields.date.date_to_datetime() to convert date into datetime... which simply makre sense
[FIX] small potential encoding issue in custom models
[FIX] purchase_requisition: converting the date into datetime using the timezone of the requisition's responsible