Refactor download methods

A local notification is now posted when all downloads are complete.

Signed-off-by: Tom Diggle <tom@tomdiggle.com>
Refactor how a episode download's progress is monitored

The cell now uses KVO instead of a timer.

Signed-off-by: Tom Diggle <tom@tomdiggle.com>
Change now displays episode title in notification if download fails

Signed-off-by: Tom Diggle <tom@tomdiggle.com>
Minor text change

Signed-off-by: Tom Diggle <tom@tomdiggle.com>