Fix #5 : Support ffmpeg 0.8 and later.

Rework ffmpeg usage to make infoqscraper compatible with ffmpeg 0.8+.

0.8.* is shipped with Ubutun 12.04 so it is important to support it.

I was not able to support 0.5 because the images are not properly scaled. It
could have been solved using PIL to resize the image but I want to get ride
of PIL eventually (not compatible with python 3).