Allow to specify custom storage for enum class

Allow to specify custom storage for enum class via:
* `self.storage=` method inside enum classes:
   
  ```ruby
  class Sex < ActiveEnum::Base
    self.storage = :i18n
  end
  ```
* `:storage` option for `enumerate` method with block:
  
  ```ruby
  class Person < ActiveRecord::Base
    enumerate :sex, storage: :i18n do
      # ...
    end
  end
  ```