fix math::is_odd for a negative argument

I think that sprout::math::is_odd(-1) should be true.

It also fixes math::pow for a case that x is a negative and y is a negative odd integer.

cf. http://melpon.org/wandbox/permlink/XVKea2I6CdVJCbZX
Merge pull request #66 from kariya-mitsuru/fix_is_odd

fix math::is_odd for a negative argument