Bugfix: Fix iOS8 issue in rzv_objectInContext:

This is a fix for #29, which appears to be an iOS 8 behavior change.  It softens up the check for a nil context, since if a background context goes away, an object reference held past the context releasing will have the managedObjectContext property nil'd out.   This allows us to continue to use the object to get the object in a different context without having to use objectID.