AddressBook::Person now fetches the unique record id and stores it.

When trying to find an existing contact, it makes more sense to be able to search by the unique record id created for that contact in the Address Book. It's guaranteed to be unique.

http://developer.apple.com/library/ios/#documentation/AddressBook/Reference/ABRecordRef_iPhoneOS/Reference/reference.html#//apple_ref/c/func/ABRecordGetRecordID

AddressBook::Person now fetches the unique record id and stores it as @uid. Also added a .find_by_uid(uid) method.