Backup and restore feed list

Use the Backup API to backup and restore the list of feed subscriptions,
i.e. the OPML file.
Restore subscriptions when the app is first started

Perform a subscription list restore when the app is started for the
first time. Use a preference setting to figure out if the app is freshly
installed or not.
Allow restore from any AntennaPod version
Restore happens by itself upon app install
Disabled downloading of episode images. closes #390

Uses too much bandwidth
Merge branch 'backup' of git://github.com/liesen/AntennaPod into liesen-backup

Conflicts:
	src/de/danoeh/antennapod/activity/MainActivity.java

Moved calls to BackupManager from PodDBAdapter to DBWriter
Changed backup API key

Now using key for 'de.danoeh.antennapod' instead of 'de.danoeh.antennapod.debug'