Added optional callback to removeDelegate

[melle]

Added optional callback that is called when the last delegate was removed

[netbe]

looks good to me , thks for the contribution :). Will let @nilsgrabenhorst check that too

[nilsgrabenhorst]

Hi Thomas, thanks for the PR! Out of curiosity:

1. What is the use case for this change?
2. Why didn't -removeDelegate: followed by -delegateCount work for you?

[melle]

Our use case here is to cancel a long running operation, that is pointless without any observer left. The check for delegateCount should be thread safe, that's why i did it that way.