Fix : Do not re-download the first playlist song cover
The getView method of the PlaylistFragment is called all the time
for the first playlist song.
Just a little trick to not re-download cover if already done for this playlist song ID.
Merge branch 'master' of https://github.com/abarisain/dmix
Merge pull request #277 from jcnoir/master

Fix : Do not re-download the first playlist song cover