Support filtering in method `label_value_pairs`

The method `label_value_pairs` can now take multiple symbol arguments
to filter the returned array of pairs by enum keys.

Example:

    User::Role.label_value_pairs(:admin, :author) == [
      ['Admin', 0], ['Author', 1]
    ]
Change description and temporarily stop using FG

- Updated the top description string for `describe`
- Temporarily changed test user creation to default Rails API
  because I'm having issues with other tests and FactoryGirl
Initial commit