Adding support for using ClassyEnum with ActiveRecord's inheritance_column

See README and specs. Or.. these examples:

```ruby
class Breed < ClassyEnum::Base; end
class Breed::GoldenRetriever < Breed; end
class Breed::Snoop < Breed; end

class Dog < ActiveRecord::Base
  classy_enum_attr :type, enum: Breed
  classy_enum_sti
end
class GoldenRetriever < Dog; end
class Snoop < Dog; end

dog.type.snoop?
dog.type == :snoop
dog.type == Breed::Snoop.new
```

```ruby
class Dog < ActiveRecord::Base
  classy_enum_attr :type, enum: Breed, :sti => true
end
```

```ruby
class Dog < ActiveRecord::Base
  classy_enum_attr :breed, enum: Breed, :sti => true
  self.inheritance_column = :breed
end
```