Fix Interval#pow as well as Polynomial#apply

Interval#pow should never return a negative bound when called with
an even exponent. Polynomial required some changes to get around
the assumpion that (x * x) == x.pow(2), which is obviously not
true for intervals.

Fixes #241.
fix bug with PolyDense#apply
fix test properties
update README with newer version
Add some extra instances for Option[_].