Make EnumInput available to formtastic-bootstrap form builder.

Hi there,

formtastic-bootstrap uses its custom namespace `FormtasticBootstrap`.
To make active_enum working with formtastic-bootstrap the `EnumInput` has to be defined in the module `FormtasticBootstrap::Inputs` and inherit from `FormtasticBootstrap::Inputs::SelectInput`.

fixes https://github.com/adzap/active_enum/issues/24