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.

include/discord-rpc.h

@@ -76,6 +76,7 @@ DISCORD_EXPORT void Discord_UpdateConnection(void);
 #endif
 
 DISCORD_EXPORT void Discord_UpdatePresence(const DiscordRichPresence* presence);
+DISCORD_EXPORT void Discord_ClearPresence();
 
 DISCORD_EXPORT void Discord_Respond(const char* userid, /* DISCORD_REPLY_ */ int reply);