Version 6.0.0

• Upgrades to Createsend API v3.3 which includes new breaking changes
• Breaking: 'client.campaigns' now returned an object to support pagination (use .Results to ge the array of campaigns)
• Added 'Tags' as another field that is returned in 'client.scheduled', 'client.drafts' and 'client.campaigns'
• Added 'Name' as another field that is returned in 'campaign.summary'
• Add new support for 'client.tags' endpoint (ie: getting list of tags for the client)
• Add support for pagination, filtering and sorting to 'client.campaigns' endpoint
• Bump rake to ~> 12.3.3
• Adding support for returning ListJoinedDate for each subscriber.
  • List.Active()
  • List.Bounced()
  • List.Unsubscribed()
  • List.Unconfirmed()
  • List.Deleted()
  • Segment.Subscribers()
  • Subscriber.Get()