Fixed metadata init when missing key in track data
String cast for all track's metadata
Artwork process refactoring
Added sc-tagger

This script provides tag process on already downloaded mp3 from
sc-syncer.
This script expect mp3 filename begin with track's id.
Added missing dependencies

sc-syncer requires stagger for id3 tags support.
Running tests depends to mock, nose and coverage packages.
Added main function for sc-syncer
Setup id3 tag after downloaded mp3 file.
Moved scripts in `scripts` directory
Added option to sc-syncer

Added --without-tag option for sc-syncer script that disable tag support
after downloaded mp3 file.
Prompt user for confirmation in sc-tagger

Confirmation needed before processing sc-tagger on directory.
sc-tagger: print useful informations for directory