Add Type property to Messages

Complies with the new MessageType property on API v6

src/Discord.Net/API/Common/Message.cs

@@ -9,6 +9,8 @@ namespace Discord.API
         public ulong Id { get; set; }
         [JsonProperty("channel_id")]
         public ulong ChannelId { get; set; }
+        [JsonProperty("type")]
+        public MessageType Type { get; set; }
         [JsonProperty("author")]
         public Optional<User> Author { get; set; }
         [JsonProperty("content")]

src/Discord.Net/Entities/Messages/IMessage.cs

@@ -17,6 +17,8 @@ namespace Discord
         string Text { get; }
         /// <summary> Gets the time this message was sent. </summary>
         DateTimeOffset Timestamp { get; }
+        /// <summary> Gets the type of message this is. Value will always be default, unless the message was sent in a group. </summary>
+        MessageType Type { get; }
 
         /// <summary> Gets the channel this message was sent to. </summary>
         IMessageChannel Channel { get; }

src/Discord.Net/Entities/Messages/Message.cs

@@ -18,6 +18,7 @@ namespace Discord
         public bool IsTTS { get; private set; }
         public string Text { get; private set; }
         public bool IsPinned { get; private set; }
+        public MessageType Type { get; private set; }
         
         public IMessageChannel Channel { get; }
         public IUser Author { get; }

src/Discord.Net/Entities/Messages/MessageType.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Discord
+{
+    public enum MessageType
+    {
+        Default = 0,
+        RecipientAdd = 1,
+        RecipientRemove = 2,
+        Call = 3,
+        NameChange = 4,
+        IconChange = 5
+    }
+}