sync: create functions to get folder and file names on device

only effects MP3PlayerDevice
easy way to get folder and filename that episode will be
synced to on device
get_episode_folder_on_device(episode):
	returns folder location on device episode will be
	synced into
get_episode_file_on_device(episode):
	returns filename that will be used on device
Merge branch 'sync_hook' of github.com:amayer5125/gpodder into sync_hook