Fanart for artist/album/song

WIth this pull request, the addon grabs the fanart for a given artist from http://www.theaudiodb.com/ and sets it accordingly (either for songs or albums). By default this option is disabled as it slows down the browsing a bit.
I created also the required strings for strings.xml.

In a few words, the addon gets the fanart from the json api and saves it into text files (in the userdata) - like name%20of%20artist.txt. The file contents is a list containing all the available fanart image urls. When adding listitems the addon only makes a new request to the api if the artist text file does't exist.  If it doesn't exist, it will grab the fanart list and save it to the file. If it exists, the addon will read the file and randomly select one of the images from the list.

Keep the good work