Update annotation card timestamps regulary

For annotation timestamps we use the fuzzyTime filter
which subtracts the annotation.updated timestamp from the current time
to show the time passed after this annotation was modified.

In the past it was updated regulary however after merging the
view-sort overhaul branch it was removed and by default an annotation
scope didn't receive any digest cycle unless user interaction happens
on that card.

So the timestamps weren't updated regulary.
This fix does nothing more than triggers a digest cycle at every
15 seconds for every annotation scope. Because filters are recalculated
at every digest scope, the timestamps at the cards are updated too.

Since annotation directive uses isolated scope the digest cycle doesn't
blow up, and nothing outside of this scope is affected.

As for the two scope.destroy functions, both of the are executed.

Fix #1615
I'm sorry to keep pushing this, but the following to me is clearer. Each function has it's own responsibility and is named clearly. You can infer what `toFuzzyString` and `nextFuzzyUpdate` are going to return. Whereas you need to look at the source of `getFuzzyObject` to work out what the object is and what properties it has. The calculation speed is moot here, I'd rather have clear code than fast code.

```
updateTimeStamp = ->
  currentTime = ctrl.annotation.updated
  nextTime = (timeHelpers.nextFuzzyUpdate(currentTime) * 1000) + 500

  scope.timestamp = dateFilter(timeHelpers.toFuzzyString(currentTime))
  $timeout(-> render(updateTimeStamp), nextTime, false)
```

If this still doesn't convince you. Then merge it.