I got this building for iOS7.

This uses dispatch groups (of single dispatches) to avoid the need for
the new API. I’ve stripped out the QOS stuff that isn’t available on
iOS7.

Haven’t managed to run the tests on iOS7 and obviously the QoS tests
are removed.
Avoid block comments so the loss of functionality is clear.
Remove block from DispatchBlock - group is held instead.
Avoid block comment so removed test is visible in diff.
Merge pull request #1 from josephlord/iOS7usingDispatchGroups

iOS7 compatible using dispatch groups