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
Extensions: Add on_episode_synced Extension Call

on_episode_synced pararms:
device: gpodder.sync.Device instance
episode: gpodder.model.PodcastEpisode instance

replace on_episode_downloaded with on_episode_synced in sync.py

This will allow users to make extensions that are triggered
after a file has been synced to a device
Merge pull request #117 from amayer5125/sync_hook

Sync: Extension Hook