[CRM] Migrate to new API

- Task: https://www.odoo.com/web#id=10543&view_type=form&model=project.task&action=333&active_id=248
- Pad: https://pad.odoo.com/p/r.qE00VdREVpboKWJp
Please refer to the ORM guide for the right use of the decorators https://www.odoo.com/documentation/8.0/reference/orm.html#module-openerp.api