Adds a generic "Social Declaration Report Module", and a French customization of these reports

erp5_payroll embeeds new modules to generate Social Declaration Reports, based of the payroll. It includes 2 modules, one to handle the different kind of reports themselves, and an other to store data related to the employee's working contracts.

erp5_payroll_l10n_fr can generate a French Social Declaration Report, named "DSN" (= Déclaration Sociale Nominative).