fixed a "bug" in itec-scenario.files
added method downloadBefore to IDownloader and implementations;
SVCWindowDownloader uses it and says: If download didnt finish until x miliseconds, 
the data is useless, and cancels the download and notifys the player.

the svc player sets the download-before time to max(bufferTimeInMS-500ms, 2000)
so there is always 2 seconds of time to download a segment.