LP singular tweak to get ordered set import working

As of Xcode 5, the Xcode-generated NSManagedObject subclasses have a different naming scheme for ordered relationships. Previously, they ended with the word "Set", but this has now been removed. This commit removes the word 'Set' from the string that's used to build a selector to get the these ordered relationships during data import, since adding that word was causing import to fail.
Merge pull request #1 from lonelyplanet/add_singular_fix

LP singular tweak to get ordered set import working