multi columns scopes

- define TenantScope as an alias on a Facade towards a shared instance of TenantScope.
- set tenant scope by addTenant(column, id) (typically in app/filters.php)
- request a $tenants variable on Eloquent model defining which column(s) should be used  as tenant-id
- move some code from model to TenantScope.php
- introduces getTenantScope in model (to be able to change it if needed)