Robustness : Avoid crash due to NowPlayingFragment{421080e8} not attached to Activity
 java.lang.IllegalStateException: Fragment NowPlayingFragment{421080e8} not attached to Activity
        at android.support.v4.app.Fragment.getResources(Fragment.java:579)
        at com.namelessdev.mpdroid.fragments.NowPlayingFragment$updateTrackInfoAsync.onPostExecute(NowPlayingFragment.java:563)
        at com.namelessdev.mpdroid.fragments.NowPlayingFragment$updateTrackInfoAsync.onPostExecute(NowPlayingFragment.java:520)
Unset activity to avoid leaks
Merge pull request #282 from jcnoir/master

Robustness : Avoid crash due to NowPlayingFragment{421080e8} not attache...