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/serialization.cpp

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