Bacon.retry source function is called after the specified delay

Previously the source function was called immediately after the first error which was quite nasty especially with an ajax source.

Also `interval` is renamed to `delay` since it's not really an interval but rather a retry specific, dynamic delay. I know this is not backwards compatible but `Bacon.retry` just landed to master so maybe we can still improve the API naming a bit :-P Furthermore, it's not even documented yet! Wait, I promised to deliver that...