Debugbrowser fix

fixes issue partly #189

The failing test is due to `math.cos(math.pi/2.0)` being `6.12303176911e-17` instead of zero.

When I do `Math.cos(Math.PI/2.0)` in the same browser it does return 0

I don't have any experience with the math stuff in skulpt so I don't know how to fix that.