Default transformer use

The Core Data [documentation on transformable attributes](https://developer.apple.com/library/mac/documentation/cocoa/conceptual/coredata/articles/cdNSAttributes.html) has this caveat:

__Important__: Although the default transformer is the transformer specified by `NSKeyedUnarchiveFromDataTransformerName`, this transformer is actually used _in reverse_. If you specify the default transformer explicitly, Core Data would use it “in the wrong direction.”

I made adjustments to the use of value transformers to account for the default case. Test cases have been added to cover the cases of using the default transformer and a custom transformer.