Use requests instead of urllib2

Many timeouts can be observed while scrapping RadioNova, let's try to
see if urllib2 is the problem.

At the same time, improve scrapper design a little bit, fix some bug,
maintain... this kind of stuff.
Pretend to be the web player

Request the server with the same headers than the web player, in an
attempt to be as discreet as possible. I don't think it makes any
difference, but it was fun to implement.
Also, we query at the same period than the web player, just to be fair
with the server.

The goal of all these changes was to reduce the amount of timeouts and
connection errors on RadioNova. But the fact is that it happens also
with the web player, so the problem comes from their server, not from
our requests.
Add Radio Grenouille scrapper

Live from Marseille !
http://www.radiogrenouille.com/
Unescape HTML entities in post-process function

Finally this functions becomes useful !
Also, we strip fields here, instead of doing it for every field inside
the scrapper.
Just playing with composition and inheritance