CATransaction Issue: Ensure that HUD hiding is always done on main thread

This fix ensures that HUD hiding is done on main thread. Leads to some nasty random crashes in my App with iOS Versions older than 7.1 because the thread that is performing the hide was deleted with uncommitted CATransactions.  
`CA_DEBUG_TRANSACTIONS = 1` shows `<Warning>: CoreAnimation: warning, deleted thread with uncommitted CATransaction; created by`