Add the ability to Get Groups from Group Type

[aryrabelo]

This PR adds:

• The ability to get Groups from Group Type GET /group_types/{id}/groups
• To fetch a collection directly (exposes the Namely::Connection#collection method

Based on: https://developers.namely.com/1.0/groups-and-teams/get-groups-from-group-type

[ramonpm]

Create a new pull request. We'll review it and leave feedback for you ASAP.

ASAP LOL! More than 1 year this is open...
Probably an abandoned project...