Allow for scope with argument

When using a Proc with a single argument as a scope, you can now reference the
instance being processed by RankedModel.

Example:

    class Duck < ActiveRecord::Base
      include RankedModel
      ranks :row_order, scope: :part_of_duck_family

      scope :part_of_duck_family,
        ->(duck) { where("ducks.family_name = ?", duck.family_name) }
    end

The above example is very simplistic, and could also be achieved with
`with_same`, but more complex queries are possible with this as well.
Fix finder when using `with_same` option. Fixes #41
Merge branch 'refs/heads/patch-2'

* refs/heads/patch-2:
  Fix finder when using `with_same` option. Fixes #41