Implemented emoji endpoints (#835)

* Implemented emoji endpoints.

* Fixed: now using API entities for REST client.

* Removed emoji listing endpoint, as per @foxbot's request.

src/Discord.Net.Core/Entities/Emotes/EmoteProperties.cs

@@ -0,0 +1,10 @@
+using System.Collections.Generic;
+
+namespace Discord
+{
+    public class EmoteProperties
+    {
+        public Optional<string> Name { get; set; }
+        public Optional<IEnumerable<IRole>> Roles { get; set; }
+    }
+}

src/Discord.Net.Core/Entities/Guilds/IGuild.cs

@@ -117,5 +117,14 @@ namespace Discord
         Task DownloadUsersAsync();
         /// <summary> Removes all users from this guild if they have not logged on in a provided number of days or, if simulate is true, returns the number of users that would be removed. </summary>
         Task<int> PruneUsersAsync(int days = 30, bool simulate = false, RequestOptions options = null);
+
+        /// <summary> Gets a specific emote from this guild. </summary>
+        Task<GuildEmote> GetEmoteAsync(ulong id, RequestOptions options = null);
+        /// <summary> Creates a new emote in this guild. </summary>
+        Task<GuildEmote> CreateEmoteAsync(string name, Image image, Optional<IEnumerable<IRole>> roles = default(Optional<IEnumerable<IRole>>), RequestOptions options = null);
+        /// <summary> Modifies an existing emote in this guild. </summary>
+        Task<GuildEmote> ModifyEmoteAsync(GuildEmote emote, Action<EmoteProperties> func, RequestOptions options = null);
+        /// <summary> Deletes an existing emote from this guild. </summary>
+        Task DeleteEmoteAsync(GuildEmote emote, RequestOptions options = null);
     }
 }