fix a warning
cleanup
note potentially useful interval algebra link
register: report intervals now include all postings in the interval

As with balance. For example, register -p 'weekly in jan' generates
these intervals: 2013/12/30-2014/01/05, 2014/01/06-2014/01/12,
2014/01/13-2014/01/19, 2014/01/20-2014/01/26, 2014/01/27-2014/02/02.
With this change, postings on 2013/12/30-31 and 2014/2/1-2 will be
included in the report, so all period totals are complete and
comparable.