Remove blocking calls causing issue with Private contexts

This commit removes thread-blocking synchronous calls to
`performBlockAndWait` that were affecting contexts with a private
concurrency type. The thread-blocking caused a cyclical deadlock when a
private context called it’s parent context.
Extract child NSManagedObject creation into helper method
Update spec tests for changed threading

This commit updates the spec tests due to the changes in threading as a
result of the private concurrency type bug fix.
Add context save after sync so managed context notification kicks off

To allow notifications to be received for the parent context upon save,
this commit adds a save block.
Merge pull request #30 from sbonami/30-Add-support-for-NSPrivateQueueConcurrencyType

Add support for NSPrivateQueueConcurrencyType