Implemented podcast offline listening

1 - Now it's possible to download the mp3 while listening and save it
to the local storage (external storage for Android). The app will use
the offline file (if it exists) when playing, so it know works when
offline.

2 - I also disable the itunes image for it ins't necessary because my
podcast only have the same cover image for all episodes.

3 - The podcast now shows it's description text as well

TODO: improve the player design and usability because of the
description text lenght.