Add scope_for_authentication method to find records

[hubertlepicki]

So I was trying to do exactly as this guy over here #723

I figured I need a scope, and I do not like the default_scope solution.

This commit uses scope_for_authentication method, if defined on given class, to find the record in question. In my case, this is implemented as where("deleted_at is not null") but I guess there are other use cases or conditions that you might want to apply in similar manner.

Please have a loook, and if you find it useful merge. If no, no harm done, I'll keep my fork for this single app for time being until I find better solution.

[ivanreese]

This looks like it'd solve an issue I'm experiencing as well — I'd like to add some includes() to the query that finds the current_user. Having a scope dedicated to Sorcery seems like it'd allow me to do this.