[IMP] fields.date.context_today: actually use the optional timestamp + better naming
[IMP] fields.date.context_today: actually use the optional timestamp + better naming

- optional datetime was not being used
- parameter renamed to timestamp to better
  indicate that it must be a datetime, not
  a simple date

akretion/openerp-server,akretion/openerp-server,akretion/openerp-server