Fix serialization issues with the isRelative property in fields

There were a couple of issues with the isRelative field:

- It was not serialized with correct name due to the naming of the getters/setters. It now serializes correctly to `isRelative` instead of `relative`
- It needed to be nullable (boolean vs Boolean) - i was having issues with iOS not recognizing it as a valid pass if isRelative was on non date fields. It would fail to open the pass.

http://developer.apple.com/library/ios/#documentation/userexperience/Reference/PassKit_Bundle/Chapters/FieldDictionary.html

Of course, it doesn't seem like setting this property made any difference on the passes that I was generating, seems to be a bug with iOS itself. Changing this allows for valid passes now though.