When class has STI, scope around subclass when polymorphic option is true.
Update the readme to document STI scoping behavior.
When class has STI, scope around subclass when polymorphic option is true.
Changed option :polymorphic => true to :scoped_by => :subclass
Changed :scoped_by => :subclass option to :by_model_type => true.
Merge pull request #117 from joe1chen/sti_subclass_scope

Add option to allow scoping around subclass when using STI.