added: bool Conv::base_round
if 'base_round' is true and 'base' is different from 2, 4, 8, or 16
and the result value is not an integer then we make an additional rounding
(after converting the last digit from the result is skipped)
changed: in Big::ToString() some additional rounding (base_round) is now made only
when the value is not an integer
git-svn-id: svn://ttmath.org/publicrep/ttmath/trunk@244 e52654a7-88a9-db11-a3e9-0013d4bc506e