7.0 account invoice unique customer ref

This PR add 2 modules: 
1/ unique_supplier_invoice_number which set a constraint on (partner_id, supplier_invoice_number) to be unique
2/ account_invoice_unique_customer_ref which set a constraint on (partner_id, name) to be unique, only for out_invoice or out_refund type account_invoice.
missing 4 spaces at the beginning of these 3 lines