Change expectations based on OS behavior.

NSDateFormatter's default year differs based on iOS version. See
http://openradar.appspot.com/12358210 for details.

Change expectations made in tests based on the default year used by
NSDateFormatter.

- Add -dateFormatterDefaultYear method to MTDatesTests.
- Use -dateFormatterDefaultYear to determine which expectation to make
  in individual tests.

This addresses 4 of 7 spec failures brought up in GitHub issue #10.

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Depending on your intentions as library maintainers, you may want to shield users from the fickleness of iOS 6. Assuming you don't, the above is a somewhat unrefined solution for the failing assertions in your tests.