Implement cache for songs
Safer import of pickle
comment
Songs are cached only if they are in a datadir
Merge branch 'master' into cache (mainly managing string encoding)

Conflicts:
	patacrep/content/song.py
	patacrep/songs.py
Using the best pickle protocol version
Ignoring cache
pylint
Solved sorting problem
Merge pull request #51 from patacrep/cache

Mise en place d'un cache