Fix dispatch_queue_t properties

[Seems](http://stackoverflow.com/q/27267865/1049134) like CocoaPods somehow prevents `dispatch_queue_t` to be objects in iOS 6+/OS X 10.8+ projects.

Seems to be related to https://github.com/CocoaPods/CocoaPods/issues/1001, https://github.com/CocoaPods/Specs/issues/1730 and https://github.com/CocoaPods/Specs/issues/4323.

Closes #410.