Supports an array of IDs

Supports an array of IDs (e.g. long[]) but uses the "..." feature in Java to allow developers to call this function without having to have an object.

E.g.
SteamApiImpl steamApi = new SteamApiImpl();
long[] longs = new long[] { 70L, 10L };
List<SteamApp> apps = steamApi.retrieveData(longs);

^ That can be simplified to:
List<SteamApp> apps = steamApi.retrieveData(70L, 10L);