didn't correctly set the exponent (when the mantisses had different size -
when 'man' was different from 'another_man')
this had impact on operator= too
sample:
Big<2,3> a = 100;
Big<3,5> b;
b = a; // b had a wrong value
git-svn-id: svn://ttmath.org/publicrep/ttmath/trunk@106 e52654a7-88a9-db11-a3e9-0013d4bc506e