Use DISPATCH_CURRENT_QUEUE_LABEL if available
Fix compilation on earlier SDKs

Check if DISPATCH_CURRENT_QUEUE_LABEL is defined.
Add comments
Suggested fixes: cleaner use of macros
Revert testing for DISPATCH_CURRENT_QUEUE_LABEL availability
It's MAX_ALLOWED, not MIN_REQUIRED
Merge pull request #156 from denivip/current-queue-ios7

Use DISPATCH_CURRENT_QUEUE_LABEL if available