Use a SyncAdapter to synchronize the directory "the Android Way"

This prepares us to do synchronizations on regular intervals (adding this is a one-liner) and also allows the user to enable and disable synchronization via the android settings. This would solve #859. Also, this is the way "all apps do synchronization in android". Or so they say...

Synchronizing the directory from the app could be done by calling this SyncAdapter, but I did not change that yet.