Dynamic Event Handler Registration (#135)

- Discord_RegisterHandlers() exported
- C# wrapper updated
- Dynamically sub/unsub to events
- Better mutex locking, for safety!

examples/button-clicker/Assets/DiscordRpc.cs

@@ -87,6 +87,9 @@ public class DiscordRpc
     [DllImport("discord-rpc", EntryPoint = "Discord_Respond", CallingConvention = CallingConvention.Cdecl)]
     public static extern void Respond(string userId, Reply reply);
 
+    [DllImport("discord-rpc", EntryPoint = "Discord_UpdateHandlers", CallingConvention = CallingConvention.Cdecl)]
+    public static extern void UpdateHandlers(ref EventHandlers handlers);
+
     public static void UpdatePresence(RichPresence presence)
     {
         var presencestruct = presence.GetStruct();