Allow payment term calculation to start at first of next month

Added new field to account.payment.term.line to make the line due date calculation start at the first of next month.

Background:
In version 6 (https://doc.odoo.com/v6.1/book/3/3_9/payment_terms/) we had a lot of features available for doing due date calculations in payment terms. In version 8 it seems a lot of this is gone. Our customer needs to give a "free month", i.e. 45 days starting at the first of the month after invoicing. For example 45 days for an invoice issued today (15/12/2014) would be February 15th 2015.

It was believed by us (wrongly it turned out) that setting the day of month parameter to -1 would produce this behaviour. It turned out it doesn't. I believe the current behaviour is counter intuitive, if not a direct bug.