SQKCoreDataOperation only saves and merges the private context it created.

Addressing #6 

BUG: After adding this test the tests suite hangs during the 
[`[SQKManagedObjectControllerTests testInitialisingWithObjectAsync]`](https://github.com/3squared/SQKDataKit/blob/824a1a125b1ab5b4fb0f14570caa005cbf18dafd/Project/SQKDataKitTests/SQKManagedObjectControllerTests.m#L254) test on the [`[_mainContext mergeChangesFromContextDidSaveNotification:notification]`](https://github.com/3squared/SQKDataKit/blob/824a1a125b1ab5b4fb0f14570caa005cbf18dafd/Classes/shared/Core/SQKContextManager.m#L81) call in SQKContextManager.