ActiveRecord scopes

Add option to define ActiveRecord scopes for each value rather than class accessors to the integer value:

```ruby
class User < ActiveRecord::Base
  as_enum :gender, [:male, :female], :scopes => true
end

women = User.female  # => ActiveRecord::Relation for Users where gender is female
men = User.male      # => ActiveRecord::Relation for Users where gender is male
```

Thoughts? If there's interest in merging this, I can add tests.