[point_of_sale] bank statement line "date" should be based on user timezone

As reported on issue 
https://github.com/odoo/odoo/issues/2199

When you apply payment in POS, it takes current time for "date" field on bank statement line, but should use context_timestamp to take care of user timezone adjustments.

Example:
If user is in time zone GMT-6:00, then after 6:00pm all bank statement lines will be recorded with date of next day, and all closing reports and related accounting will be wrong!