Store Account IDs instead of Steam IDs

Steam IDs are different in the Source 2009 engine and the new P2/CSGO engine. We must store account ids instead of steam ids to offer Counter-Strike: Global Offensive support.

```
GetAccountIDFromAuthString(String:authString[])
{
decl String:toks[3][16];

ExplodeString(authString, ":", toks, sizeof(toks), sizeof(toks[]));

new odd = StringToInt(toks[1]);
new halfAID = StringToInt(toks[2]);

return (halfAID*2) + odd;
}
```

(source: [itemlogger](http://hg.limetech.org/droplog/raw-file/13f677e0998d/item_logger.sp))