only send notifications if a user has access to the node

@greggles: if this is still open I'll take a look soon... btw (before digging in), does this cover the case where users can create subscriptions to content types they don't have access to? Anyway, i'll check this out - but curious about this case

oops - greggles - scratch my comment above (it's way too early in the morning - I was actually looking for this case in the notifications module... I'm using both on the same site :p )