Fix warnings

* DDLog.h:529:1: No 'assign', 'retain', or 'copy' attribute is specified - 'assign' is assumed
* DDLog.h:529:1: Default property attribute 'assign' not appropriate for non-GC object
Set dispatch_queue_t as strong
Try another fix to support both iOS 5+ and iOS 6+ deployment targets

http://stackoverflow.com/a/24846028/1049134
Merge pull request #397 from rivera-ernesto/fix_warnings

Fix warnings