Allow specify additional params outside of param group

Especially useful for hash params. For example:

    def_param_group :user do
      param :user, Hash do
        param :name, String
      end
    end

    param_group :user
    param :user, Hash do
      param :password, String
    end
Merge pull request #99 from iNecas/merge_params

Allow specify additional params outside of param group