Extended strack's metadata for id3 tags.

Tests: JSON data are now located into tests.ssyncer.mock_data for
readability.
Use crop format for artworks.

More infos here:
http://developers.soundcloud.com/docs/api/reference#artwork_url
Mapped id3 TextFrame and URLFrame with metadata

metadata/frames:
`created-at` - `TYER` - `TDAT` - `TIME`
`duration` - `TLEN`
`original-content-size` - `TSIZ`
`tags-list` - `TIT3`
`permalink` - `TOFN`
`genre` - `TCON`
`title` - `TIT2`
`description` - `TIT1`
`license` - `TCOP`
`uri` - `WOAS`
`permalink-url` - `WOAF`
`username` - `TPUB`
`user-url` - `WOAR`
Write id3 tags

Removed id3v24 invalid frame (TYER, TDAT, TIME, TSIZ)
Added TDOR frame for date
Added TOPE frame for artist
Process artwork tag
Fixed artist frame (TPE1) and added album (TALB).
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
Updated README

Improved Requirements, Installation sections
Added "Tag support" section
Added "ID3 Tag support" in Features section
Check filename format in sc-tagger.

Track ID is necessary, and the only way to get it is from the
filename. Added checks before begin tagging process.
Set album (TALB) to "USERNAME Soundcloud tracks"