Added Pagination for GetCampaigns and Overloads to maintain compatibility with previous arguments.

This pull request adds pagination to GetCampaigns and also corrects some confusing language around unsubscribing contacts. With the addition, ALL instances of GetCampaigns now return a ResultSet object instead of an IList. To retrieve the actual list, you can fetch the Results member which is an IList. In addition, you can pass the ResultSet member Meta.Pagination back to ConstantContact.GetCampaigns as a single parameter to fetch the next page of results.