Add correct reference time to getOffset() calls in DateColumnLocalDateMapper

See Issue #5.

This solution passes the argument Date/LocalDate's value as the argument to DateTimeZone.getOffset() to avoid
the result's dependency on DateTimeUtils.currentTimeMillis().  

The threetenbp version also looks suspect.  If you agree I can add a corresponding fix.

I'm starting to work on a unit test.  If you have feedback on how best to do that, let me know.


This PR fixes #5.