Added ability to scope the slug

[graemenelson]

I added the ability to scope the slug with the option key validate_uniqueness_scope, it works like the normal validates_uniqueness_of scope option.

http://ar.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#M000086

I also renamed the test from test_slug.rb to slug_test.rb so the "rake test" will work, and updated the gemspec definition to reflect this name change.

[tamaloa]

Merging those improvements would be great - slug is just what I was looking for (explicitly do not want a slug history option as in friendly_id) except for the scope ability missing