Adjust timestamp-difference to address DST boundary

With the current implementation of local-time, the timestamp-duration+ and timestamp-duration- operations will adjust the resulting timestamp is on the other side of  a DST boundary.  However timestamp-difference does not take DST into account.  This results in the following, which feels inconsistent to me.

> (test-diff @2014-11-02T00:00:00.000000-05:00  @2014-11-03T00:00:00.000000-06:00)
@2014-11-03T01:00:00.000000-06:00
> (test-diff @2015-03-08T00:00:00.000000-06:00 @2015-03-09T00:00:00.000000-05:00)
@2015-03-08T23:00:00.000000-05:00

After the proposed change the results are as follows:

> (test-diff @2014-11-02T00:00:00.000000-05:00  @2014-11-03T00:00:00.000000-06:00)
@2014-11-03T00:00:00.000000-06:00
> (test-diff @2015-03-08T00:00:00.000000-06:00 @2015-03-09T00:00:00.000000-05:00)
@2015-03-09T00:00:00.000000-05:00