Dynamic Event Handler Registration (#135)

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

src/serialization.h

@@ -47,6 +47,8 @@ size_t JsonWriteRichPresenceObj(char* dest,
                                 const DiscordRichPresence* presence);
 size_t JsonWriteSubscribeCommand(char* dest, size_t maxLen, int nonce, const char* evtName);
 
+size_t JsonWriteUnsubscribeCommand(char* dest, size_t maxLen, int nonce, const char* evtName);
+
 size_t JsonWriteJoinReply(char* dest, size_t maxLen, const char* userId, int reply, int nonce);
 
 // I want to use as few allocations as I can get away with, and to do that with RapidJson, you need