add Discord_ClearPresence() (#104)

send-presence example updated - start a line with 'c' to toggle
whether presence information is sent or not.

Added ClearPresence to .cs bindings

Added ClearPresence to UE4 blueprint class.

src/discord-rpc.cpp

@@ -325,6 +325,11 @@ extern "C" DISCORD_EXPORT void Discord_UpdatePresence(const DiscordRichPresence*
     SignalIOActivity();
 }
 
+extern "C" DISCORD_EXPORT void Discord_ClearPresence()
+{
+    Discord_UpdatePresence(nullptr);
+}
+
 extern "C" DISCORD_EXPORT void Discord_Respond(const char* userId, /* DISCORD_REPLY_ */ int reply)
 {
     // if we are not connected, let's not batch up stale messages for later

src/serialization.cpp

@@ -102,6 +102,7 @@ size_t JsonWriteRichPresenceObj(char* dest,
             WriteKey(writer, "pid");
             writer.Int(pid);
 
+            if (presence != nullptr)
             {
                 WriteObject activity(writer, "activity");