Audit Logs implementation (#1055)

* Copy audit logs impl from old branch and clean up

I suck at using git, so I'm gonna use brute force.

* Remove unnecessary TODOs

Category channels do not provide any new information, and the other
I forgot to remove beforehand

* Add invite update data, clean up after feedback

* Remove TODOs, add WebhookType enum for future use

WebhookType is a future-use type, as currently audit logs are the only
thing which may return it.

src/Discord.Net.Rest/API/Common/AuditLogChange.cs

@@ -0,0 +1,17 @@
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+
+namespace Discord.API
+{
+    internal class AuditLogChange
+    {
+        [JsonProperty("key")]
+        public string ChangedProperty { get; set; }
+
+        [JsonProperty("new_value")]
+        public JToken NewValue { get; set; }
+
+        [JsonProperty("old_value")]
+        public JToken OldValue { get; set; }
+    }
+}