move all models to the subfolder app/models
Moved de definition of errors to a single file and added UnknownAttribute error
Add Model#== method to be able to compare NSManagedObjects stored in a different NSMAnagedObjectContext
Instantiated models are now inserted in an internal temporal NSManagedObjectContext, which parent context is the main NSManagedObjectContext. When a model is saved, all its changed are pushed to the main context, which is then saved to persist the changes to the database. More info: http://www.cocoanetics.com/2012/07/multi-context-coredata/
Nested models created through mass-assigned nested attributes are inserted in its parent's temporal NSManagedObjectContext. This is required to set the relations, because two NSManagedObjects cannot be related if they are not in the same NSManagedObjectContext
This change fixes finders_spec.rb tests. Read the comment for more information.