Tag cover view with artist + album before download cover to avoid displaying wrong cover.
- When the cover has been downloaded the resulting cover is displayed only if it matches the tag set in the album cover view
   It is useful for the nowPlaying fragment which displayed the last downloaded cover even if it was a previous played song
Robustness
Avoid NPE when notifying user whereas getResources() returns null
Avoid NPE in playlist fragment by storing the activity in an attribute on activity creation (the getActivity() method returns sometimes null after a rotate or on wakeup)
Revert fix to not re-download album cover
Refrains the view to be refreshed sometimes whereas it should be
Robustness
Avoid NPE when notifying user whereas getResources() returns null
Avoid NPE in playlist fragment by storing the activity in an attribute on activity creation (the getActivity() method returns sometimes null after a rotate or on wakeup)
Robustness
Avoid : ERROR/AndroidRuntime(26128): FATAL EXCEPTION: MPDStatusMonitor
                java.lang.IndexOutOfBoundsException: Invalid index 0, size is 0
                at java.util.ArrayList.throwIndexOutOfBoundsException(ArrayList.java:255)
                at java.util.ArrayList.set(ArrayList.java:481)
                at org.a0z.mpd.MPDPlaylist.refresh(MPDPlaylist.java:229)
                at org.a0z.mpd.MPDPlaylist.playlistChanged(MPDPlaylist.java:172)
                at org.a0z.mpd.MPDStatusMonitor.run(MPDStatusMonitor.java:122)
Robustness => Avoid : java.lang.IllegalStateException: Content view not yet created
                at android.support.v4.app.ListFragment.ensureList(ListFragment.java:328)
                at android.support.v4.app.ListFragment.setSelection(ListFragment.java:198)
                at com.namelessdev.mpdroid.fragments.PlaylistFragment$3.run(PlaylistFragment.java:320)
Cover improve : Reset the nowplaying cover when looking for one
Do not update the playlist item view if not necessary
Merge pull request #281 from jcnoir/master

Tag cover view with artist + album before download cover to avoid displaying wrong cover.