Register upgrade (#2)

* Update init to take an optional Steam ID. Add register for steam game. Remove url from cmd line params to launched game.

* Start on a build script

include/discord-rpc.h

@@ -51,7 +51,8 @@ typedef struct DiscordEventHandlers {
 
 DISCORD_EXPORT void Discord_Initialize(const char* applicationId,
                                        DiscordEventHandlers* handlers,
-                                       int autoRegister);
+                                       int autoRegister,
+                                       const char* optionalSteamId);
 DISCORD_EXPORT void Discord_Shutdown();
 
 /* checks for incoming messages, dispatches callbacks */