Added extension methods for WebSocket->Channels

For users importing `Discord.WebSocket.Extensions`, there are now non-async extensions for GetUser.

These methods point to the appropriate method on the respective SocketgChannel.

src/Discord.Net/WebSocket/Extensions/ChannelExtensions.cs

@@ -0,0 +1,48 @@
+using System;
+
+namespace Discord.WebSocket.Extensions
+{
+    public static class ChannelExtensions
+    {
+        public static IUser GetUser(this IDMChannel channel, ulong id)
+            => GetSocketDMChannel(channel).GetUser(id);
+
+        public static IGroupUser GetUser(this IGroupChannel channel, ulong id)
+            => GetSocketGroupChannel(channel).GetUser(id) as IGroupUser;
+
+        public static IGuildUser GetUser(this ITextChannel channel, ulong id)
+            => GetSocketTextChannel(channel).GetUser(id);
+
+        public static IGuildUser GetUser(this IVoiceChannel channel, ulong id)
+            => GetSocketVoiceChannel(channel).GetUser(id);
+
+        internal static SocketDMChannel GetSocketDMChannel(IDMChannel channel)
+        {
+            var socketChannel = channel as SocketDMChannel;
+            if (socketChannel == null)
+                throw new InvalidOperationException("This extension method is only valid on WebSocket Entities");
+            return socketChannel;
+        }
+        internal static SocketGroupChannel GetSocketGroupChannel(IGroupChannel channel)
+        {
+            var socketChannel = channel as SocketGroupChannel;
+            if (socketChannel == null)
+                throw new InvalidOperationException("This extension method is only valid on WebSocket Entities");
+            return socketChannel;
+        }
+        internal static SocketTextChannel GetSocketTextChannel(ITextChannel channel)
+        {
+            var socketChannel = channel as SocketTextChannel;
+            if (socketChannel == null)
+                throw new InvalidOperationException("This extension method is only valid on WebSocket Entities");
+            return socketChannel;
+        }
+        internal static SocketVoiceChannel GetSocketVoiceChannel(IVoiceChannel channel)
+        {
+            var socketChannel = channel as SocketVoiceChannel;
+            if (socketChannel == null)
+                throw new InvalidOperationException("This extension method is only valid on WebSocket Entities");
+            return socketChannel;
+        }
+    }
+}

src/Discord.Net/WebSocket/Extensions/GuildExtensions.cs

@@ -4,7 +4,7 @@ using System.Linq;
 
 namespace Discord.WebSocket.Extensions
 {
-    // TODO: Docstrings
+    // Todo: Docstrings
     public static class GuildExtensions
     {
         // Channels