Ability to initialise SQKContextManager with an NSPersistentStoreCoordinator

This is useful if you want more control of the `NSPersistentStoreCoordinator ` the context manager uses. For example you may have more than a single persistent store configuration which the existing initialiser [assumes by passing nil in the NSPersistentStoreCoordinator inialisiation](https://github.com/3squared/SQKDataKit/blob/62d6aff22aadad29816019e14de820c4a36b8fb4/Classes/shared/ContextManager/NSPersistentStoreCoordinator%2BSQKAdditions.m#L33).