Get rid of the PIL dependency

PIL was used to convert PNG into JPEG files to feed ffmpeg because older releases
of ffmpeg had issue with PNG.

It seems now OK to accept PNG too. Affected user can download a static linked
version of ffmpeg as a workaround.

Removing PIL is important because it prevented the port of infoqscraper to python 3.
Do some code cleanup in the downloader.

Removed some useless legacy code and fixed documentation.
Add Python 3 support.

infoqscraper is now compatible with Python 2 and Python 3 (>= 2.6).
Comptability is ensured by the six module.

Currently only Python 2.7 and Python 3.3 have be tested. I plan to
use travis to run a wider test campaign.