multi columns scopes 

- define "TenantScope" as an alias on a Facade towards a shared instance of a TenantScope.
- introduces getTenantScope in model (can be useful to ne)
- set tenant scope by addTenant(column, id) method (typically to be use in app/filters.php)
- mandatory variable $tenantAttribute in Eloquent model defining which attribute(s) should be used to define tenants
- move some code from model to TenantScope.php