Add specs for Rustle::Color. Fix serious bug in hsb factory method.

Andy's sister here.  I saw Andrew mention this project on Facebook and decided to check it out.  I like to write specs when reading code because it helps me understand it better (and frees me up to do a bit of refactoring). So I did wrote at least a couple specs for most of methods of the `Color` class.

Along the way I discovered the `Rustle::Color.hsb` method was giving significantly wrong results because  of integer division happening where float division was needed.