Slovenian subtitles

I added Slovenian subtitles.

I put the srt file in the root directory since the materials dir is not on github (or did I miss it?) -- this has to be moved to the appropriate dir.

[Boštjan Pirnar](https://bitcointalk.org/index.php?action=profile;u=48856) and his team have also created a version of the What is Bitcoin video with great Slovenian voice-over:
https://www.youtube.com/watch?feature=player_embedded&v=LrLzaeQpCRc . Maybe this link can be added somewhere? I didn't find an obvious appropriate place though.