IPHONEAPP-588
IPHONEAPP-588

Dispatch all story prune requests onto the background thread so that they are properly executed in order. Also adjust the sort ordering of the stories. They will now be sorted by post date, then title. Finally, the most recent featured story will be moved to the top.

MIT-Mobile/MIT-Mobile-for-iOS,xNUTs/MIT-Mobile-for-iOS,MIT-Mobile/MIT-Mobile-for-iOS,xNUTs/MIT-Mobile-for-iOS,smartcop/MIT-Mobile-for-iOS,smartcop/MIT-Mobile-for-iOS