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/AuditLogOptions.cs

@@ -0,0 +1,27 @@
+using Newtonsoft.Json;
+
+namespace Discord.API
+{
+    internal class AuditLogOptions
+    {
+        //Message delete
+        [JsonProperty("count")]
+        public int? MessageDeleteCount { get; set; }
+        [JsonProperty("channel_id")]
+        public ulong? MessageDeleteChannelId { get; set; }
+
+        //Prune
+        [JsonProperty("delete_member_days")]
+        public int? PruneDeleteMemberDays { get; set; }
+        [JsonProperty("members_removed")]
+        public int? PruneMembersRemoved { get; set; }
+
+        //Overwrite Update
+        [JsonProperty("role_name")]
+        public string OverwriteRoleName { get; set; }
+        [JsonProperty("type")]
+        public string OverwriteType { get; set; }
+        [JsonProperty("id")]
+        public ulong? OverwriteTargetId { get; set; }
+    }
+}