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.
Merge pull request #11 from modocache/fix-ios5-tests-defaultyear

Change expectations based on OS behavior.