Add support for day of month to the cron rules

The cron-style playing rules was missing day of month, which meant
that, for example, you couldn't specify things like "play this on
the 1st of each month"

Signed-off-by: Fredrik Lindberg <fli@shapeshifter.se>
Make sure track is playable when resuming playback

When resuming playback, make sure the current track is in a playlist
that is currently playable and if it's not, select a new track.  This
makes sure that date controlled playlists are properly obeyed during
long pauses.

Signed-off-by: Fredrik Lindberg <fli@shapeshifter.se>
Keep the internal playing state consistent

Set playing to false on end-of-stream before calling next/play.
Not doing this would cause play() to be called twice since next()
also called play() if playing is set to true.

Signed-off-by: Fredrik Lindberg <fli@shapeshifter.se>