campaigns/template-content support and small changes to Subscribe method

1. Returns the support of custom dictionary for the MergeVars parameter of Subscribe. Old code still works fine.

        [System.Runtime.Serialization.DataContract]
        public class MyMergeVar : MergeVar
        {
             [System.Runtime.Serialization.DataMember(Name = "FNAME")]
             public string FirstName { get; set; }
             [System.Runtime.Serialization.DataMember(Name = "LNAME")]
             public string LastName { get; set; }
         }
         
         MailChimpManager mc = new MailChimpManager("YourApiKeyHere-us2");
         EmailParameter email = new EmailParameter()
         {
             Email = "customeremail@righthere.com"
         };
         MyMergeVar myMergeVars = new MyMergeVar();
         myMergeVars.Groupings = new List&lt;Grouping&gt;();
         myMergeVars.Groupings.Add(new Grouping());
         myMergeVars.Groupings[0].Id = 1234; // replace with your grouping id
         myMergeVars.Groupings[0].GroupNames = new List&lt;string&gt;();
         myMergeVars.Groupings[0].GroupNames.Add("Your Group Name");
         myMergeVars.FirstName = "Testy";
         myMergeVars.LastName = "Testerson";
         EmailParameter results = mc.Subscribe(strListID, email, myMergeVars); // Works
         EmailParameter results = mc.Subscribe(strListID, email, new Dictionary<string,string>{{"FNAME", "Yagg"}, {"LNAME", "Adder"}, {"mc_language", "ru"}}); // Works too.

2. campaigns/template-content support