Ability to replace or disable default scope on load

[roxblnfk]

https://github.com/cycle/orm/issues/51

$select->load('comments', ['load' => new PublicCmments()])

Now it works so that the new Scope (Constraint) is added to a default global Scope.

It would be very cool to be able to replace or disable a default Scope

[wolfy-j]

See other tickets as well, we are going to use new parameter "scope" to replace currently active scope/constrain.