File scanning moved to background thread.

>IMO, if we want to nail this once and for all, the best would be to throw the file scanning into a background processing (threaded) that would populate the file list while it has already started running.
-https://github.com/brunobraga/termsaver/pull/15#issuecomment-33288350

I've refactored file scanning & animation into separate threads, so that animation may begin as soon as a single file path is put in the queue by the file scanning thread. For the programmer screensaver run  with  my path to [torvalds/linux](https://github.com/torvalds/linux), the delay between hitting enter and the start of animation is imperceptible.

Mission Success.