No need for dependency graph to be in one queue

From https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationObjects/OperationObjects.html (look for “Configuring Interoperation Dependencies”):

> Dependencies are also not limited to operations in the same queue. Operation objects manage their own dependencies and so it is perfectly acceptable to create dependencies between operations and add them all to different queues.

I have to admit that I have used such behaviour only once some time ago (producers-consumers situation, and I need to keep them under different concurrency constraints), so maybe you have included that warning for some reason I don’t know.
Merge pull request #182 from drodriguez/patch-1

No need for dependency graph to be in one queue