Don't use nested contexts for importing

[kevinrenskers]

Just read a very interesting article about importing large sets of data into Core Data. Turns out that the nested background context solution that NLCoreData is also advocating is pretty shitty. The old style non-nested contexts and mergeChangesFromContextDidSaveNotification: seems to be performing a LOT better.

Setting up this kind of Core Data stack is boring so it would be perfect if NLCoreData would handle this. I'm curious to hear your thoughts on this.